Class 9 Chapter 7 - Study of Gas Laws Exercise Ex. 7

Question Num 1

What will be the minimum pressure required to compress 500 dm³ of air at 1 bar to 200 dm³ temperature remaining constant.

Solution Num 1

V₁ = 500 dm³

P₁ = 1 bar

T₁ = 273 K

V₂ = 500 dm³

T₂ = 273 K

P₂= ?

Question Num 2

2 litres of a gas is enclosed in a vessel at a pressure of 760 mmHg. If temperature remains constant, calculate pressure when volume changes to 4 dm³.

Solution Num 2

V = 2 litres

P = 760 mm

V₁ = 4000 m³ [1 dm³ = 4 litres]

P₁= ?

Question Num 3

At constant temperature, the effect of change of pressure on volume of a gas was as given below:

1. Plot the following graphs

1. P vs V

2. P vs 1/V

3. PV vs P

Interpret each graph in terms of a law.

2. Assuming that the pressure values given above are correct, find the correct measurement of the volume. 

Solution Num 3

 i. P vs. V:

At constant temperature, P is inversely proportional to V. Thus, the plot of V versus P will be a rectangular hyperbola.

 ii. P vs. 1/V:

According to Boyle's law, at constant temperature, pressure of a fixed amount of gas varies inversely to its volume. The graph of pressure verses 1/V shows a positive slope.

 iii. PV vs. P:

According to Boyle's law, the product of pressure and volume is constant at constant temperature. The graph of PV versus P is constant which indicates that the given gas obeys Boyle's law.

2. The correct measurements of the volume are given below:

Question Num 4

800 cm³ of gas is collected at 650 mm pressure. At what pressure would the volume of the gas reduce by 40% of its original volume, temperature remaining constant?

Solution Num 4

Given: 

V = 800 cm³

P = 650 m

P₁= ?

V₁ = reduced volume = 40% of 800

 =

Net V₁ = 800 - 320 = 480 cm³  

T = T₁

Using the gas equation,

Since T = T₁

Question Num 5

A cylinder of 20 litres capacity contains a gas at 100 atmospheric pressure. How many flasks of 200 cm³capacity can be filled from it at 1 atmosphere pressure, temperature remaining constant?

Solution Num 5

Question Num 6

A steel cylinder of internal volume 20 litres is filled with hydrogen at 29 atmospheric pressure. If hydrogen is used to fill a balloon at 1.25 atmospheric pressure at the same temperature, what volume will the gas occupy?

Solution Num 6

V = 20 litre  

P = 29 atm

P₁ = 1.25 atm 

V₁ =? 

T = T₁

Question Num 7

561 dm³ of a gas at STP is filled in a 748 dm³ container. If temperature is const

ant, calculate the percentage change in pressure required.

Solution Num 7

Initial volume = V₁ = 561 dm³

Final volume = V₂ = 748 dm³

Difference in volume = 748 - 561 = 187 dm³

As the temperature is constant,

Decrease in pressure percentage =

Question Num 8

88 cm³ of nitrogen is at a pressure of 770 mm mercury. If the pressure is raised to 880 mmHg, find by how much the volume will diminish, temperature remaining constant.

Solution Num 8

V = 88 cm³ 

P = 770 mm

P₁ = 880 mm 

V₁= ? 

T = T₁

Volume diminishes = 88 - 77 = 11 cm³  

Question Num 9

A gas at 240 K is heated to 127°C. Find the percentage change in the volume of the gas (pressure remaining constant).

Solution Num 9

Let volume = 100 ml

T = 240 K

Volume increased = x ml

New volume = 100 + x ml

T₁ = 400 K

Question Num 10

Certain amount of a gas occupies a volume of 0.4 litre at 17°C. To what temperature should it be heated so that its volume gets (a) doubled, (b) reduced to half, pressure remaining constant?

Solution Num 10

(a) V₁ = 0.4 L 

     V₂ = 0.4 × 2L

     T₁ = 17°C (17 + 273) = 290 K

     T₂= ?

(b) V₁ = 0.4 L 

     V₂ = 0.2 L 

     T₁ = 17°C (17 + 273) = 290 K

     T₂= ?

Question Num 11

A gas occupies 3 litres at 0°C. What volume will it occupy at -20°C, pressure remaining constant?

Solution Num 11

V = 3 litres 

P = P₁

V₁= ? 

T = 0°C = 0 + 273 = 273 K

T₁ = -20°C = -20°C + 273 = 253 K

Question Num 12

A gas occupies 500 cm³ at normal temperature. At what temperature will the volume of the gas be reduced by 20% of its original volume, pressure being constant?

Solution Num 12

V = 500 cm³ 

Normal temperature, t = 0°C = 0 + 273 K

V₁ = Reduced volume + 20% of 500 cm³

Net, V₁  = 500 - 100 = 400 cm³

T₁= ?

P = P₁

Question Num 13

Calculate the final volume of a gas 'X' if the original pressure of the gas at STP is doubled and its temperature is increased three times.

Solution Num 13

V₁ = X

P₁ = 1 atm

V₂= ?

T₂ = 3 T₁

P₂ = 2 atm

Question Num 14

A sample of carbon dioxide occupies 30 cm³ at 15°C and 740 mm pressure. Find its volume at STP.

Solution Num 14

V = 30 cm³ 

P = 740 mm

T = 288 K

P₁ = 760 mm

V₁= ? 

T₁ = 273 K

Question Num 15

50 cm³ of hydrogen is collected over water at 17°C and 750 mmHg pressure. Calculate the volume of a dry gas at STP. The water vapour pressure at 17°C is 14 mmHg.

Solution Num 15

V = 50 cm³ 

P = 750 - 14 = 736 mm

T = 290 K

P₁ = 760 mm

V₁= ? 

T₁ = 273 K

Question Num 16

At 0°C and 760 mmHg pressure, a gas occupies a volume of 100 cm³. Kelvin temperature of the gas is increased by one-fifth and the pressure is increased one and a half times. Calculate the final volume of the gas.

Solution Num 16

V = 100 cm³ 

P = 760 mm

T = 273 K

V₁= ? 

Question Num 17

It is found that on heating a gas its volume increases by 50% and its pressure decreases to 60% of its original value. If the original temperature was -15°C, find the temperature to which it was heated.

Solution Num 17

Let the original volume (V) = 1 and

the original pressure (P) = 1 and

the temperature given (T) = -15°C  = -15 + 273 = 258 K

V₁ or new volume after heating = original volume + 50% of original volume

P₁ or decreased pressure = 60%

T₁ = to be calculated

Question Num 18

A certain mass of a gas occupies 2 litres at 27°C and 100 Pa. Find the temperature when volume and pressure become half of their initial values.

Solution Num 18

V = 2 litres

P = 100 Pa

T = 300 K

T₁ = 75 - 273 = -198°C

Question Num 19

2500 cm³ of hydrogen is taken at STP. The pressure of this gas is further increased by two and a half times (temperature remaining constant). What volume will hydrogen occupy now?

Solution Num 19

V₁ = 2500 cm³

P₁ = 1 atm = 760 mm

T₁ = 273 K

V₂= ?

T₂ = 273 K

Question Num 20

Taking the volume of hydrogen as calculated in Q.19, what change must be made in Kelvin (absolute) temperature to return the volume to 2500 cm³ (pressure remaining constant)?

Solution Num 20

V₁ = 714.29 cm³

P₁ = P₂ = P

T₁ = 273 K

V₂ = 2500 cm³

T₂= ?

Question Num 21

A given amount of gas A is confined in a chamber of constant volume. When the chamber is immersed in a bath of melting ice, the pressure of the gas is 100 cmHg.

a. What is the temperature when the pressure is 10 cmHg?

b. What will be the pressure when the chamber is brought to 100°C

Solution Num 21

1. V₁ = V₂ = V

       P₁ = 100 cmHg

       T₁ = 273 K

       P₂ = 10 cmHg

       T₂= ?

2. V₁ = V₂ = V

       P₁ = 100 cmHg

       P₂= ?

       T₁ = 273 K

       T₂ = 373 K

Question Num 22

A gas is to be filled from a tank of capacity 10,000 litres into cylinders each having capacity of 10 litres. The condition of the gas in the tank is as follows:

1. Pressure inside the tank is 800 mmHg.
2. Temperature inside the tank is -3°C.

When the cylinder is filled, the pressure gauge reads 400 mmHg and the temperature is 0°C. Find the number of cylinders required to fill the gas.

Solution Num 22

Capacity of the cylinder V = 10000 litres

P = 800 mm

T = -3°C = -3 + 273 = 270 K P₁ = 400 mmHg

T₁ = 0°C = 0 + 273 = 273 K

V₁= ?

Question Num 23

Calculate the volume occupied by 2 g of hydrogen at 27°C and 4 atmosphere pressure if at STP it occupies 22.4 litres.

Solution Num 23

V₁ = 22.4 litres

P₁ = 1 atm

T₁ = 273 K

V₂ =?

T₂ = 300 K

P₂ = 4 atm

Question Num 24

What temperature would be necessary to double the volume of a gas initially at STP if the pressure is decreased to 50%?

Solution Num 24

V₁ = V₁

P₁ = 760 atm

T₁ = 273 K

V₂ = 2V₁

T₂ =?

Question Num 25

Which will have greater volume when the following gases are compared at STP:

1. 1.2/N₂ at 25°C and 748 mmHg
2. 1.25/O₂ at STP 

Solution Num 25

1. V = 1.2 litres

       P = 748 mmHg

       T = 298 K

       P₁ = 760 mmHg

       T₁ = 273 K

       V₁= ?

2. V = 1.25 litres

      P = 760 mmHg

      T = 273 K

      P₁ = 760 mmHg

      T₁ = 273 K

      V₁= ?

     1.25 litres O₂ will have greater volume than 1.2 litres N₂. 

Question Num 26

Calculate the volume of dry air at STP that occupies 28 cm³ at 14°C and 750 mmHg pressure when saturated with water vapour. The vapour pressure of water at 14°C is 12 mmHg.

Solution Num 26

Pressure due to dry air,

P = 750 - 12 = 738 mm

V = 28 cm³

T = 14°C = 14 + 273 = 287 K

P₁ = 760 mmHg

V₁= ?

T₁ = 0°C = 273 K

Using gas equation,

Question Num 27

An LPG cylinder can withstand a pressure of 14.9 atmosphere. The pressure gauge of the cylinder indicates 12 atmosphere at 27°C. Because of a sudden fire in the building, the temperature rises. At what temperature will the cylinder explode?

Solution Num 27

P = 14.9 atm

V = 28 cm³

T = ?

P₁ = 12 atm

V = V₁

T₁ = 300 K

Using gas equation,

Question Num 28

22.4 litres of a gas weighs 70 g at STP. Calculate the weight of the gas if it occupies a volume of 20 litres at 27°C and 700 mmHg of pressure.

Solution Num 28

Step 1:

V₁ = 20 litres

P₁ = 700 mm

T₁ = 300 K

V₂= ?

T₂ = 273 K

P₂ = 760 mm

Step 2:

22.4 litres of the gas at STP weighs = 70 g

16.76 litres of the gas has weight at STP =

Question 1

What do you understand by gas?

Solution 1

Gas is a state of matter in which interparticle attraction is weak and interparticle space is so large that the particles become completely free to move randomly in the entire available space.

Question 2

Give the assumptions of the kinetic molecular theory.

Solution 2

1. Gases are made of tiny particles which move in all possible directions at all possible speeds. The size of molecules is small as compared to the volume of the occupied gas.
2. There is no force of attraction between gas particles or between the particles and the walls of the container. So, the particles are free to move in the entire space available to them.
3. The moving particles of gas collide with each other and with the walls of the container. Because of these collisions, gas molecules exert pressure. Gases exert the same pressure in all directions.
4. There is large interparticle space between gas molecules, and this accounts for high compressibility of gases.
5. Volume of a gas increases with a decrease in pressure and increase in temperature.
6. Gases have low density as they have large intermolecular spaces between their molecules.
7. Gases have a natural tendency to mix with one and other because of large intermolecular spaces. So, two gases when mixed form a homogeneous gaseous mixture.
8. The intermolecular space of a gas is reduced because of cooling. Molecules come closer resulting in liquefaction of the gas. 

Question 3

During the practical session in the lab when hydrogen sulphide gas having offensive odour is prepared for some test, we can smell the gas even 50 metres away. Explain the phenomenon.

Solution 3

The phenomenon is diffusion. In air, gas molecules diffuse to mix thoroughly. Hence, we can smell hydrogen sulphide gas from a distance in the laboratory.

Question 4

What is diffusion? Give an example to illustrate it.

Solution 4

Diffusion is the process of gradual mixing of two substances, kept in contact, by molecular motion.

Example:

If a jar of chlorine is opened in a large room, the odorous gas mixes with air and spreads to every part of the room. Although chlorine is heavier than air, it does not remain at the floor level but spreads throughout the room.

Question 5

How is molecular motion related with temperature?

Solution 5

Temperature affects the kinetic energy of molecules. So, molecular motion is directly proportional to temperature.

Question 6

State (i) the three variables for gas laws and (ii) SI units of these variables.

Solution 6

1. Three variables for gas laws: Volume (V), Pressure (P), Temperature (T)
2. SI units of these variables:

For volume: Cubic metre (m³)

For pressure: Pascal (Pa)

For temperature: Kelvin (K) 

Question 7

1. State Boyle's Law.
2. Give its

 i. Mathematical expression

 ii. Graphical representation and

 iii. Significance 

Solution 7

1. Boyle's law: At constant temperature, the volume of a definite mass of any gas is inversely proportional to the pressure of the gas.  

Or

Temperature remaining constant, the product of the volume and pressure of the given mass of a dry gas is constant

i. Mathematical representation: 

According to Boyle's Law,

where K is the constant of proportionality.

If V' and P' are some other volume and pressure of the gas at the same temperature, then

 ii. Graphical representation of Boyle's Law:

1.  : Variation in volume (V) plotted against (1/P) at a constant temperature: A straight line passing through the origin is obtained.

2. V vs P: Variation in volume (V) plotted against pressure (P) at a constant temperature: A hyperbolic curve in the first quadrant is obtained.

3. PV vs P: Variation in PV plotted against pressure (P) at a constant temperature: A straight line parallel to the X-axis is obtained.

 iii. Significance of Boyle's law:

According to Boyle's law, on increasing pressure, volume decreases. The gas becomes denser. Thus, at constant temperature, the density of a gas is directly proportional to the pressure.

At higher altitude, atmospheric pressure is low so air is less dense. As a result, lesser oxygen is available for breathing. This is the reason mountaineers carry oxygen cylinders. 

Question 8

Explain Boyle's Law on the basis of the kinetic theory of matter.

Solution 8

Boyle's law on the basis of the kinetic theory of matter:

• According to the kinetic theory of matter, the number of particles present in a given mass and the average kinetic energy is constant.
• If the volume of the given mass of a gas is reduced to half of its original volume, then the same number of particles will have half the space to move.
• As a result, the number of molecules striking the unit area of the walls of the container at a given time will double and the pressure will also double.
• Alternatively, if the volume of a given mass of a gas is doubled at constant temperature, the same number of molecules will have double the space to move.
• Thus, the number of molecules striking the unit area of the walls of a container at a given time will become one-half of the original value.
• Thus, pressure will also get reduced to half of the original pressure. Hence, it is seen that if the pressure increases, the volume of a gas decreases at constant temperature, which is Boyle's law. 

Question 9

The molecular theory states that the pressure exerted by a gas in a closed vessel results from the gas molecules striking against the walls of the vessel. How will the pressure change if

1. The temperature is doubled keeping the volume constant
2. The volume is made half of its original value keeping the T constant 

Solution 9

1. Pressure will double.
2. Pressure remains the same. 

Question 10

1. State Charles's law.
2. Give its

 i. Graphical representation

 ii. Mathematical expression and

 iii. Significance 

Solution 10

Charles's Law

At constant pressure, the volume of a given mass of a dry gas increases or decreases by 1/273^rd of its original volume at 0°C for each degree centigrade rise or fall in temperature.

At temperature T₁ (K) and volume V₁ (cm³):

 ...(i)

At temperature T₂ (K) and volume V₂ (cm³):

 ….(ii)

From (i) and (ii),

For Temperature = Conversion from Celsius to Kelvin

1 K = °C + 273

Example:

20°C = 20 + 273 = 293 K

Graphical representation of Charles's law

T vs V: The relationship between the volume and the temperature of a gas can be plotted on a graph. A straight line is obtained.

Significance of Charles's Law: The volume of a given mass of a gas is directly proportional to its temperature; hence, the density decreases with temperature. This is the reason that

(a) Hot air is filled in balloons used for meteorological purposes. (b) Cable wires contract in winters and expand in summers. 

Question 11

Explain Charles's law on the basis of the kinetic theory of matter.

Solution 11

Charles's law on the basis of the kinetic theory of matter:

According to the kinetic theory of matter, the average kinetic energy of gas molecules is directly proportional to the absolute temperature. Thus, when the temperature of a gas is increased, the molecules would move faster and the molecules will strike the unit area of the walls of the container more frequently and vigorously. If the pressure is kept constant, the volume increases proportionately. Hence, at constant pressure, the volume of a given mass of a gas is directly proportional to the temperature (Charles's law). 

Question 12

Define absolute zero and absolute scale of temperature. Write the relationship between °C and K.

Solution 12

Absolute zero

The temperature -273°C is called absolute zero.

Absolute or Kelvin scale of temperature

The temperature scale with its zero at -273°C and each degree equal to one degree on the Celsius scale is called Kelvin or the absolute scale of temperature.

Conversion of temperature from Celsius scale to Kelvin scale and vice versa

The value on the Celsius scale can be converted to the Kelvin scale by adding 273 to it.

Example:

20°C = 20 + 273 = 293 K 

Question 13

1. What is the need for the Kelvin scale of temperature?
2. What is the boiling point of water on the Kelvin scale? Convert it into centigrade scale. 

Solution 13

1. The behaviour of gases shows that it is not possible to have temperature below 273.15°C. This act has led to the formulation of another scale known as the Kelvin scale. The real advantage of the Kelvin scale is that it makes the application and the use of gas laws simple. Even more significantly, all values on the Kelvin scale are positive.
2. The boiling point of water on the Kelvin scale is 373 K. Now, K = °C + 273 and °C = K - 273.

The Kelvin scale can be converted to the degree Celsius scale by subtracting 273 So, the boiling point of water on the centigrade scale is 373 K - 273 = 100°C. 

Question 14

1. Define STP or NTP.
2. Why is it necessary to compare gases at STP? 

Solution 14

1. Standard or Normal Temperature and Pressure (STP or NTP)

The pressure of the atmosphere which is equal to 76 cm or 760 mm of mercury and the temperature is 0°C or 273 K is called STP or NTP. The full form of STP is Standard Tempe rature and Pressure, while that of NTP is Normal Temperature and Pressure.

Value: The standard values chosen are 0°C or 273 K for temperature and 1 atmospheric unit (atm) or 760 mm of mercury for pressure.

The standard values chosen are 0°C or 273 K for temperature and 1 atmospheric unit (atm) or 760 mm of mercury for pressure.

2. The volume of a given mass of dry enclosed gas depends on the pressure of the gas and the temperature of the gas in Kelvin, so to express the volume of the gases, we compare these to STP. 

Question 15

Correct the following statements:

1. Volume of a gas is inversely proportional to its pressure at constant temperature.
2. Volume of a fixed mass of a gas is directly proportional to its temperature, pressure remaining constant.
3. 0°C is equal to zero Kelvin.
4. Standard temperature is 25°C.
5. Boiling point of water is 273 K. 

Solution 15

1. Volume of a gas is directly proportional to the pressure at constant temperature.
2. Volume of a fixed mass of a gas is inversely proportional to the temperature, the pressure remaining constant.
3. -273°C is equal to zero Kelvin.
4. Standard temperature is 0°C.
5. The boiling point of water is -373 K. 

Question 16

1. What is the relationship between the Celsius and Kelvin scales of temperature?
2. Convert (i) 273°C to Kelvin and (ii) 293 K to °C. 

Solution 16

Temperature on the Kelvin scale (K)

= 273 + temperature on the Celsius scale

Or K = 273 + °C

(i) 273°C in Kelvin

    t°C = t K - 273

    273°C = t K - 273

    t K = 273 + 273 = 546 K

    ∴ 273°C = 546 K

(ii) 293 K in °C

    t°C = 293 - 273

    t°C = 20°C

    ∴ 293 K = 20°C 

Question 17

State the laws which are represented by the following graphs:

Solution 17

1. Charles's law
2. Boyle's law 

Question 18

Give reasons for the following:

1. All temperature in the absolute (Kelvin) scale are in positive figures.
2. Gases have lower density compared to solids or liquids.
3. Gases exert pressure in all directions.
4. It is necessary to specify the pressure and temperature of a gas while stating its volume.
5. Inflating a balloon seems to violate Boyle's law.
6. Mountaineers carry oxygen cylinders with them.
7. Gas fills the vessel completely in which it is kept. 

Solution 18

1. The advantage of the Kelvin scale is that it makes the application and use of gas laws simple. Of more significance is that all values on the scale are positive, removing the problem of negative (-) values on the Celsius scale.
2. The mass of a gas per unit volume is small because of the large intermolecular spaces between the molecules. Therefore, gases have low density. In solids and liquids, the mass is higher and intermolecular spaces are negligible.
3. At a given temperature, the number of molecules of a gas striking against the walls of the container per unit time per unit area is the same. Thus, gases exert the same pressure in all directions.
4. Because the volume of a gas changes remarkably with a change in temperature and pressure, it becomes necessary to choose a standard value of temperature pressure.
5. According to Boyle's law, the volume of a given mass of a dry gas is inversely proportional to its pressure at constant temperature.

When a balloon is inflated, the pressure inside the balloon decreases, and according to Boyle's law, the volume of the gas should increase. But this does not happen. On inflation of a balloon along with reduction of pressure of air inside the balloon, the volume of air also decreases, violating Boyle's law.

6. Atmospheric pressure is low at high altitudes. The volume of air increases and air becomes less dense because volume is inversely proportional to density. Hence, lesser volume of oxygen is available for breathing. Thus, mountaineers have to carry oxygen cylinders with them.
7. Interparticle attraction is weak and interparticle space is large in gases because the particles are completely free to move randomly in the entire available space and takes the shape of the vessel in which the gas is kept. 

Question 19

How did Charles's law lead to the concept of absolute scale of temperature?

Solution 19

1. The temperature scale with its zero at -273°C and where each degree is equal to the degree on the Celsius scale is called the absolute scale of temperature.
2. The temperature -273°C is called absolute zero. Theoretically, this is the lowest temperature which can never be reached. All molecular motion ceases at this temperature.
3. The temperature -273°C is called absolute zero.

Question 20

What is meant by aqueous tension? How is the pressure exerted by a gas corrected to account for aqueous tension?

Solution 20

Gases such as nitrogen and hydrogen are collected over water as shown in the diagram. When the gas is collected over water, the gas is moist and contains water vapour. The total pressure exerted by this moist gas is equal to the sum of the partial pressures of the dry gas and the pressure exerted by water vapour. The partial pressure of water vapour is also known as aqueous tension.

  P_total = P_gas + P_{water vapour}

 P_gas = P_total- P_{water vapour}

Actual pressure of gas = Total pressure - Aqueous tension 

Question 21

State the following:

1. Volume of a gas at 0 Kelvin
2. Absolute temperature of a gas at 7°C
3. Gas equation
4. Ice point in absolute temperature
5. STP conditions 

Solution 21

1. Volume of gas is zero.
2. Absolute temperature is 7 + 273 = 280 K.
3. The gas equation is

4. Ice point = 0 + 273 = 273 K 
5. Standard temperature is taken as 273 K or O°C.  

Standard pressure is taken as 1 atmosphere (atm) or 760 mmHg. 

Question 22

Choose the correct answer:

1. The graph of PV vs P for a gas is

1. Parabolic
2. Hyperbolic
3. A straight line parallel to the X-axis
4. A straight line passing through the origin

2. The absolute temperature value that corresponds to 27°C is

1. 200 K
2. 300 K
3. 400 K
4. 246 K

3. Volume-temperature relationship is given by

1. Boyle
2. Gay-Lussac
3. Dalton
4. Charles

4. If pressure is doubled for a fixed mass of a gas, its volume will become

1. 4 times
2. ½ times
3. 2 times
4. No change 

Solution 22

1. (iii) Straight line parallel to the X-axis.
2. (ii) 27°C = 27 + 273 = 300 K
3. (iv) Charles
4. (ii) 1/2 times 

Question 23

Match the following:

Solution 23

Question 24

Write the value of

1. Standard temperature in

1. °C
2. K

2. Standard pressure in

1. atm 
2. mmHg
3. cmHg 
4. torr 

Solution 24

1. °C = O°C
2. K = 273 K

1. 1 atm
2. 760 mmHg
3. 76 cmHg
4. 1 torr = 133.32 Pascal 

Question 25

1. The average kinetic energy of the molecules of a gas is proportional to the ………….
2. The temperature on the Kelvin scale at which molecular motion completely ceases is called……………
3. If temperature is reduced to half, ………….. would also reduce to half.
4. The melting point of ice is …………. Kelvin. 

Solution 25

1. Absolute temperature
2. Absolute zero
3. Volume
4. 273 

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(A)

What is the contribution of the following in atomic structure?

a. Maharshi Kanada

b. Democritus

Solution 1

1. According to Maharshi Kanada:
   1. Matter consisted of indestructible particles called paramanus (now called atoms).
   2. A paramanu does not exist in the free state, rather it combines with other paramanus to form a bigger particle called the anu (now called a molecule).
2. Democritus called paramanu as atom which comes from the Greek word atomos, meaning indivisible. 

Question 2

State Dalton's atomic theory.

Solution 2

Main postulates of Dalton's atomic theory:

1. Matter consists of very small and indivisible particles called atoms.
2. Atoms can neither be created nor be destroyed.
3. The atoms of an element are alike in all respects, but they differ from the atoms of other elements.
4. Atoms of an element combine in small numbers to form molecules.
5. Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.
6. Atoms are the smallest units of matter which can take part in a chemical reaction. 

Question 3

What is an α (alpha) particle?

Solution 3

An α-particle is a doubly charged helium ion (He²⁺) containing two protons and two neutrons. It is formed by removing two electrons from the helium atom.

Question 4

What are cathode rays? How are these rays formed?

Solution 4

Cathode rays are the beam of electrons which travel from the negatively charged end (i.e. cathode to anode) of a vacuum tube, across a voltage difference between the electrodes placed at each end.

Formation of cathode rays: 

Generally gases are poor conductors of electricity. However, when a high voltage charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm of mercury), the gases become good conductors of electricity and begin to flow in the form of rays.

These rays are called cathode rays and they travel from the cathode towards the anode.

Question 5

What is the nature of the charge on

1. Cathode rays and
2. Anode rays?

Solution 5

1. Cathode rays are negatively charged as they consist of negatively charged particles called electrons.
2. Anode rays are made of positively charged particles.

Question 6

How are X-rays produced?

Solution 6

When a beam of cathode rays is made to fall upon hard metallic targets like tungsten, X-rays are produced.

Question 7

Why are anode rays also called as 'canal rays'?

Solution 7

When perforated rays were used in the discharge tube, another set of rays travelling in a direction opposite to that of the cathode rays, i.e. from the anode towards the cathode, is seen, as in the figure below. These rays are called canal rays because they passed through the holes or canals in the cathode. These rays were named positive rays or anode rays.

Question 8

How does cathode ray differ from as anode rays?

Solution 8

Question 9

State one observation which shows that atom is not indivisible.

Solution 9

Chadwick discovered neutral particles present in an atom by bombarding light nuclei like beryllium with alpha particles, i.e. helium nuclei.

₄Be⁹ + ₂He⁴  → ₆C¹² + ₀n¹

Discovery of the neutron also showed that an atom is divisible.

Question 10

a. Name an element which does not contain neutron

b. If an atom contains one electron and one proton, will it carry as a whole is neutral

Solution 10

a. Hydrogen does not contain any neutron.

b. No. It is neutral.

Question 11

On the basis of Thomson's model of an atom explain how an atom as a whole is neutral.

Solution 11

Thomson's Atomic Model:

According to Thomson's atomic model, an atom is made of positively charged substances in the form of a sphere. Electrons are embedded into this sphere, and the total positive charge of the sphere is equal to the total negative charge of electrons, and hence, the atom remained electrically neutral. This model was not accepted because it was unable to explain how positively charged particles were shielded from negatively charged particles without getting neutralised.

Question 12

Which sub-atomic particle was discovered by

a. Thomson

b. Goldstein

c. Chadwick

Solution 12

a. Thomson discovered particles.

b. Goldstein discovered protons.

c. Chadwick discovered neutrons.

Question 13

Name the sub-atomic particle whose charge is:

a. +1

b. -1

c. 0

Solution 13

a. Proton

b. Electron

c. Neutron

Question 15

On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?

Solution 15

According to Rutherford's model of an atom, the subatomic particle in a nucleus is a proton.

Question 16

Which part of atom was discovered by Rutherford?

Solution 16

The nucleus was discovered by Rutherford.

Question 17

How was it shown that atom has empty space?

Solution 17

Rutherford performed an experiment by allowing a stream of alpha particles to pass through a very thin gold foil. He observed that alpha particles pass through the metal foil without deviating from their path. This shows that an atom contains a large empty space called nuclear space.

Question 18

State one major drawback of Rutherford's model

Solution 18

One major drawback of Rutherford's model was the comparison of electrons with the planets in the solar system.

Thus, when an electron moved around the nucleus continually, it should radiate energy, i.e. lose energy. As a result it should be gradually pulled towards the nucleus and end up colliding with it. This should result in the total collapse of the atom.

However, we know that the atom is structurally stable. Thus, Rutherford's model could not explain this stability.

Question 19

In the figure given alongside

a. Name the shells denoted by A,B, and C. Which shell has least energy

b. Name X and state the charge on it

c. The above sketch is of …………. Model of an atom

Solution 19

a)  A is for K shell or I shell.

      B is for L shell or II shell.

      C is for M shell or III shell.

      Shell K has the minimum amount of energy.

b) X is a nucleus; it is positively charged.

c)  The above sketch is of Bohr model of an atom. 

Question 20

Give the postulates of Bohr's atomic model

Solution 20

1. Atom has a central nucleus surrounded by electrons.
2. The electrons revolve around the nucleus in definite circular paths called orbits.
3. Each orbit has fixed energy. Therefore, these orbits are also known as energy levels or energy shells.
4. These orbits or energy levels are represented either by a number 1, 2, 3,... known as the principal quantum number (n) of the orbit or by K, L, M,....
5. There is no change of energy of electrons as long as they keep revolving in the same energy level and the atom remains stable. Electrons can jump from the higher orbit to the lower orbit when they lose energy and jump to the next when they gain energy. 

 4 B excercise

Question 1

1. Name the three fundamental particles of an atom. 
2. Give the symbol and charge of each particle. 

Solution 1

1. The three fundamental particles of an atom are electron proton and neutron. 
2.  

Question 2

Complete the table given below by identifying P, Q, R and S.

Solution 2

Question 3

The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What are its atomic number and mass number?

Solution 3

Atomic number = Number of protons or number of electrons = 4

Mass number = Number of protons + Number of neutrons = 4 + 5 = 9

Question 4

The atomic number and mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement?

Solution 4

Atomic number (11) of sodium conveys information that the number of protons and electrons is the same.

Mass number (23) of sodium indicates the sum of protons and neutrons.

Question 5

Write down the names of the particles represented by the following symbols and explain the meaning of superscript and subscript numbers attached

Solution 5

p = proton

n = neutron

e = electron

Superscript number : These superscript number show their mass number

Subscript number : These numbers show their atomic number

Question 6

From the symbol , state the mass number, the atomic number and electronic configuration of magnesium.

Solution 6

Mass number = 24

Atomic number = 12

No. of electrons = 24 - 12 = 12

Electronic configuration = 2, 8, 2

Question 7

Sulphur has an atomic number 16 and a mass of 32.

State the number of protons and neutrons in the nucleus of sulphur. Give a simple diagram to show the arrangement of electrons in an atom of sulphur.

Solution 7

Atomic number = 16

Atomic mass = 32

Number of protons = 16

Number of electrons = 16

Number of neutrons = 32 - 16 = 16

Electronic configuration = 2, 8, 6

Question 8

Explain the rule according to which electrons are filled in various energy levels.

Solution 8

1. The maximum capacity of a shell to accommodate electrons is given by the general formula 2n², where n is the serial number of a shell.
2. The maximum number of electrons possible in the outermost shell is 8 and that in the penultimate shell is 18.
3. It is not necessary for an orbit to be completed before another is formed. In fact, a new orbit is formed when the outermost shell attains 8 electrons.

Question 9

Draw the orbital diagram of ion and state the number of three fundamental particles present in it.

Solution 9

Orbital diagram of

Atomic number of Ca²⁺ is 18.

Electronic configuration of Ca²⁺ is 2, 8, 8.

Number of three fundamental particles of Ca²⁺:

Protons: 18

Electrons: 18

Neutrons: 40 - 18 = 22

Question 10

Write down the electronic configuration of the following:

Write down the number of electrons in X and neutrons in Y.

Solution 10

Electronic configuration: ²⁷X₁₃ = 2, 8, 3

Number of electrons in X = 13

Number of neutrons in X = 27 - 13 = 14

Electronic configuration of ³⁵Y₁₇ = 2, 8, 7

Number of electrons in Y = 17

Number of neutrons in Y = 35 - 17 = 18

Hence, formula ²⁷X₁₃ stands for ²⁷Na₁₃ and ³⁵Y₁₇ for ³⁵Cl₁₇

Compound formula = NaCl

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(C)

Question 1

How does the Modern atomic theory contradict and correlate with Dalton's atomic theory?

Solution 1

The latest research on the atom has proved that most of the postulates of Dalton's atomic theory contradict. However, Dalton was right that atoms take part in chemical reactions.

Comparisons of Dalton's atomic theory with the modern atomic theory.

Dalton's atomic theory:

1. Atoms are indivisible.
2. Atoms of the same element are similar in every respect.
3. Atoms combine in a simple whole number ratio to form molecules.
4. Atoms of different elements are different.
5. Atoms can neither be created nor be destroyed.

Modern atomic theory:

1. Atoms are no longer indivisible and consist of electrons, protons, neutrons and even more sub-particles.
2. Atoms of the same element may differ from one another called isotopes.
3. Atoms of different elements may be similar called isobars.
4. Atoms combine in a ratio which is not a simple whole number ratio; e.g. in sugar, the C₁₂H₂₂O₁₁ ratio is not a whole number ratio.

Question 2

1. What are inert elements?
2. Why do they exist as monoatoms in molecules?
3. What are valence electrons? 

Solution 2

1. The elements have a complete outermost shell, i.e. 2 or 8 electrons. They ordinarily do not enter into any reaction.
2. These exist as monoatoms because molecules of these elements contain only one atom.
3. Valence electrons: The number of electrons present in the valence shell is known as valence electrons. 

Question 3

In what respects do the three isotopes of hydrogen differ? Give their structures.

Solution 3

The three isotopes differ only due to their mass number which is respectively 1, 2 and 3 and named protium, deuterium and tritium.

Question 4

Match the atomic numbers 4,14,8,15 and 19 with each of the following:

1. A solid non-metal of valency 3.
2. A gas of valency 2.
3. A metal of valency 1.
4. A non-metal of valency 4. 

Solution 4

Question 5

Draw diagrams representing the atomic structures of the following:

1. Sodium atom
2. Chlorine ion
3. Carbon atom
4. Oxygen ion 

Solution 5

1. Sodium atom

2. Chlorine atom

3. Carbon atom

4. Oxygen ion 

Question 6

What is the significance of the number of protons found in the atoms of different elements?

Solution 6

The number of protons in the nucleus of an atom determines the element type of the atom. 

Question 7

Elements X, Y and Z have atomic numbers 6,9 and 12 respectively. Which one:

1. Forms an onion
2. Forms a cation
3. Has four electrons in its valence shell? 

Solution 7

Atomic numbers of X          Y            Z

                               6          9           12

                            (2, 4)   (2, 7)     (2, 8, 2)

1. Y(2, 7) forms an anion.
2. Z(2, 8, 2) forms a cation.
3. X(2, 4) has four electrons in the valence shell.

Question 8

Element X has electronic configuration 2,8,18,8,1. Without identifying X,

1. Predict the sign and charge on a simple ion of X.
2. Write if X will be an oxidizing agent or a reducing agent. Why? 

Solution 8

1. X¹⁺
2. Oxidising agent, because it has the ability to donate electrons. 

Question 9

Define the terms:

1. Mass number
2. Ion
3. Cation
4. Anion
5. Element
6. orbit 

Solution 9

1. Mass number is the sum of the number of protons and the number of neutrons in the nucleus of an atom.
2. Ion is an atom or molecule which carries a positive or negative charge because of loss or gain of electrons.
3. Cation is a positively charged ion which is formed when an atom loses one or more electrons; for example, Na⁺, Hg²⁺ and Ca²⁺.
4. Anion is a negatively charged ion which is formed when an atom gains one or more electrons; for example, Cl^- and I^-.
5. Element is a substance which cannot be split up into two or more simple substances by usual chemical methods of applying heat, light or electric energy; for example, hydrogen, oxygen and chlorine.
6. Orbit is a circular path around the nucleus in which electrons of the atom revolve. 

Question 10

From the symbol  for the element helium, write down the mass number and the atomic number of the element.

Solution 10

Atomic number = 2

Mass number = 4

Question 11

Five atoms are labeled A to E

1. Which one of these atoms:
   1. contains 7 protons
   2. has electronic configuration 2,7

2. Write down the formula in the compound formed between C and D
3. Predict : (i) metals (ii) non-metals 

Solution 11

a.  

1. Atom E contains 7 protons.
2. Atom B has an electronic configuration 2, 7.

b. Atom C stands for ₇Li³, Atom D stands for ₈O¹⁶.

    Hence, compound formula is Li₂O.

c. Metals: A and C, Non-metals: B, D, E 

Question 12

An atom of an element has two electrons in the M shell.

What is the (a) atomic number (b) number of protons in this element?

Solution 12

Number of electrons in the M shell = 2

So, the number of electrons in the K and L shells will be = 2, 8

Hence, atomic number = 2 + 8 + 2 = 12

Number of protons = 12

Question 13

Solution 13

 i.  

Hence, composition of nuclei

₁₂Mg²⁴ →

 ii. Electronic configuration = 2, 8, 2

2. Mass numbers of two isotopes of magnesium are different because of different number of neutrons, i.e. 12 and 14, respectively. 

Question 14

What are nucleons? How many nucleons are present in phosphorus? Draw its structure.

Solution 14

Nucleons: Particles which constitute the nucleus are called nucleons.

Protons and neutrons are the nucleons.

Atomic mass of phosphorus = 31

Atomic number = 15

Question 15

What are isotopes? With reference to which fundamental particle do isotopes differ? Give two uses of isotopes.

Solution 15

Isotopes: Atoms of the same element having the same atomic number but different mass numbers.

Isotopes differ with reference to neutrons.

Uses of isotopes:

1. Some isotopes are radioactive, i.e. isotopes of cobalt are used for treating cancer and other diseases.
2. An isotope of ²³⁵U is used as a fuel in a nuclear reactor.

Question 16

Why do  have the same chemical properties? In what respect do these atoms differ?

Solution 16

Only electrons take part in chemical reactions. Chemical properties depend on the electronic configuration. Isotopes of the element Cl  have the same atomic number, and hence, the same configuration. So, they have the same chemical properties. These differ only in physical contents and weights because neutrons contribute to the mass of an atom.  have different number of neutrons 18 and 20, respectively.

Question 17

Explain fractional atomic mass. What is the fractional mass of chlorine?

Solution 17

Atomic masses of the isotopes of chlorine are 35 and 37. However, in any given sample of chlorine gas, the isotopes occur in the approximate ratio 3:1, 75% of Cl³⁵ and 25% of Cl³⁷. So, the relative atomic mass or atomic weight of chlorine is 35.5.

Fractional atomic mass of chlorine

At. Mass = 3(35) + 1(37) = 105 + 37

                          2                    2

Question 18

a. What is meant by 'atomic number of an element''?

b. Complete the table given below

c. Write down the electronic configuration of (i) chlorine atom (ii) chlorine ion

Solution 18

a. Atomic number of an element is the number of protons in the nucleus of an atom.

b.  

c.  

 i. Electronic configuration of chlorine = 2, 8, 7

 ii. Electronic configuration of chlorine ion = 2, 8, 8 

Question 19

Name the following:

1. The element which does not contain any neutron in its nucleus.
2. An element having valency 'zero'
3. Metal with valency 2
4. Two atoms having the same number of protons and electrons but different number of neutrons.
5. The shell closest to the nucleus of an atom 

Solution 19

1. Hydrogen
2. Helium
3. Magnesium
4. Hydrogen and carbon
5. K 

Question 20

Give reasons

1. Physical properties of isotopes are different.
2. Argon does not react.
3. Actual atomic mass is greater than mass number.
4.  

Solution 20

1. Physical properties depend on atomic mass, and isotopes have different mass number, i.e. they have different number of neutrons. So, isotopes have different physical properties.
2. Argon does not react as it has the outermost orbit complete, i.e. 8 electrons in the outermost shell.
3. Actual atomic mass is greater than the mass number because the mass number is a whole number approximation of atomic mass unit. In fact, neutrons are slightly heavier than protons, and an atom has over 200 sub-atomic particles.
4. are isotopes of chlorine element which differ in the number of neutrons, whereas chemical properties are determined by the electronic configuration of an atom. Isotopes of an element are chemically alike. 

Question 21

An element A atomic number 7 mass numbers 14

B electronic configuration 2,8,8

C electrons 13, neutrons 14

D Protons 18 neutrons 22

E Electronic configuration 2,8,8,1

State (i) Valency of each element (ii) which one is a metal (iii) which is non-metal (iv) which is an inert gas

Solution 21

(i)

Element A

Atomic number = 7 = Number of electrons = 2, 5

Valency of A = 8 - 5 = 3

Element B

Electronic configuration 2, 8, 8

Valency of B = Zero

Element C has 13 electrons

Electronic configuration = 2, 8, 3

Valency of C = 3

Element D

Protons = 18 = Electrons = 2, 8, 8

Valency of D = Zero

Element E

Electronic configuration = 2, 8, 8, 1

Valency of E = 1

(ii) C and E are metals.

(iii) A is a non-metal.

(iv) A, C and E are not inert gases.

Question 22

Choose the correct option

1. Rutherford's alpha-particle scattering experiment discovered

A. Electron

B. Proton

C. Atomic nucleus

D. Neutron

2. Number of valence electrons in O^2- is :

A. 6

B. 8

C. 10

D. 4

3. Which of the following is the correct electronic configuration of potassium?

A. 2,8,9

B. 8,2,9

C. 2,8,8,1

D. 1,2,8,8 

Solution 22

1. Atomic nucleus
2. [6]
3. [2, 8, 8, 1] 

Question 23

Explain

a. Octet rule for formation of sodium chloride

b. Duplet rule for formation of hydrogen 

Solution 23

Elements tend to combine with one another to attain the stable electronic configuration of the nearest inert gas.

(a) Sodium chloride

Sodium atom has 1 electron in the valence shell which it donates to the chlorine atom with 7 electrons in the valence shell to attain the stable electronic configuration of the nearest inert gas, i.e. 8 electrons in the valence shell. This is known as the octet rule. These elements combine to form sodium chloride.

(b) Hydrogen

Hydrogen atom has one electron in the valence shell which it shares with another hydrogen atom having one electron to complete its duplet state, i.e. two electrons in the valence shell and resulting in the formation of hydrogen.

Question 24

Complete the following table relating to the atomic structure of some elements.

Solution 24

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(D)

Question 1

Question 1

How do atoms attain noble gas configurations?

Solution 1

Combining atoms attain the noble gas configuration by transferring or sharing electrons.

Question 2

Define electrovalent bond.

Solution 2

The chemical bond formed due to the electrostatic force of attraction between a cation and an anion is called an electrovalent bond. 

Question 3

Elements are classified as metals, non-metal, metalloids and inert gases. Which of them form electrovalent bond?

Solution 3

Metals have a tendency to lose their valence electrons (1, 2 or 3). So, they combine with non-metals which have 7, 6 or 5 electrons in their valence shell to form an electrovalent bond.

Question 4

1. An atom X has three electrons more than the noble gas configuration. What type of ion will it form?
2. Write the formula of its (X)  

 i. sulphate

 ii. nitrate

 iii. phosphate

 iv. carbonate

 v. hydroxide. 

Solution 4

1. An atom X will lose its three electrons to acquire the noble gas configuration and form a positive ion (a cation).
2. The formulae are

 i. X(SO₄)₃

 ii. X(NO₃)₃

 iii. XPO₄

 iv. X₂(CO₃)₃

 v. X(OH)₃ 

Question 5

Mention the basic tendency of an atom which makes it to combine with other atoms

Solution 5

Besides chaos, everything in this world wants stability. The same is the case with atoms. For atoms, stability means having the electron arrangement of an inert gas, i.e. octet in the outermost shell. Helium has two electrons (duplet), while all other inert gases, i.e. neon, argon, krypton, xenon and radon have eight electrons (octet) in the outermost shell.

Question 6

What type of compounds are usually formed between metals and non-metals and why?

Solution 6

Electrovalent compounds are usually formed between metals and non-metals.

Atoms of metallic elements which have 1, 2 or 3 valence electrons can lose electron(s) to atoms of non-metallic elements which have 5, 6 or 7 valence electrons and thereby form an electrovalent bond.

Question 7

1. In the formation of the compound XY₂, an atom X gives one electron to each Y atom. What is the nature of bond in XY₂?
2. Draw the electron dot structure of this compound. 

Solution 7

1. Nature of bond in XY₂ is an ionic bond.
2. Electron dot structure of XY₂:

Question 8

An atom X has 2, 8, 7 electrons in its shell. It combines with Y having 1 electron in its outermost shell.

1. What type of bond will be formed between X and Y?
2. Write the formula of the compound formed. 

Solution 8

1. Ionic bond
2. XY 

Question 9

Draw orbit structure diagram of sodium chloride (NaCl) and calcium oxide (CaO).

Solution 9

NaCl

MgCl₂

CaO

Question 10

Compare :

1. Sodium atom and sodium ion
2. Chlorine atom and chloride ion, with respect to

           i. Atomic structure

          ii. Electrical state

Solution 10

1. In chlorine atom, the number of protons (17) is equal to the number of electrons (17).

               In chloride ion, there are 17 protons but 18 electrons.

2. Chlorine atom is electrically neutral. Chloride ion is negatively charged.
3. Chlorine atom is reactive. It reacts with sodium vigorously forming sodium chloride. Chloride ion is unreactive. It does not react with sodium.
4. Chlorine (Cl₂) is a poisonous, toxic, corrosive gas which is used in the manufacture of bleaching agents and disinfectants. It is non-toxic and readily adsorbed by plants. 

Question 11

The electronic configuration of fluoride ion is the same as that of neon atom. What is the difference between the two?

Solution 11

Fluoride ion is a negatively charged ion with 9 protons and 10 electrons.

Neon atom is electrically neutral with 10 protons and 10 electrons.

Question 12

1. What do you understand by redox reactions?
2. Explain oxidation and reduction in terms of loss or gain of electrons.

Solution 12

1. Transfer of electron(s) is involved in the formation of an electrovalent bond. The electropositive atom undergoes oxidation, while the electronegative atom undergoes reduction. This is known as a redox reaction.
2. Oxidation: In the electronic concept, oxidation is a process in which an atom or ion loses electron(s).
   Zn → Zn²⁺ + 2e^-
   Reduction: In the electronic concept, reduction is a process in which an atom or ion accepts electron(s).
   Cu²⁺ + 2e^- → Cu

Question 13

Potassium (at No.19) and chlorine (at No.17) react to form a compound. Explain on the basis of electronic concept:

 i. Oxidation

 ii. Reduction

 iii. Oxidizing agent

 iv. Reducing agent 

Solution 13

2K + Cl₂ → 2KCl

 i. Oxidation: In the electronic concept, oxidation is a process in which an atom or ion loses electron(s).

K → K⁺ + e^-

 ii. Reduction: In the electronic concept, reduction is a process in which an atom or ion accepts electron(s).

Cl₂ + 2e^-→ 2Cl^-

 iii. Oxidising agent

An oxidising agent oxidises other substances either by accepting electrons or by providing oxygen or an electronegative ion, or by removing hydrogen or an electropositive ion.

Cl₂ + 2e^-→ 2Cl^-

 iv. Reducing agent

A reducing agent reduces other substances either by providing electrons or by providing hydrogen or an electropositive ion, or by removing oxygen or an electronegative ion.

K → K⁺ + e^-

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(E)

Question 1

1. Define covalent bond.
2. Give an example of the covalent bond formed by

(i) Similar atoms (ii) Dissimilar atoms 

Solution 1

1. The chemical bond formed between two combining atoms by mutual sharing of one or more pairs of electrons is called a covalent bond.
2. Covalent bond formed by

1. Bond formed between two Cl atoms; Cl‒Cl
2. Bond formed between hydrogen atom and chlorine atom; H‒Cl  

Question 2

Covalent bonds can be single, double or triple covalent bonds. How many electrons are shared in each? Give an example of each type.

Solution 2

A single covalent bond is formed by sharing of one pair of electrons between atoms, each atom contributing one electron.

A double bond is formed by sharing of two pairs of electrons between two atoms.

A triple bond is formed by sharing of three pairs of electrons between two atoms.

Question 3

Show number of bonds in

(i) ethene molecule  (ii) ethyne molecule

Solution 3

1. Ethene molecule has one double covalent bond and four single covalent bonds.

2. Ethyne molecule has one triple covalent bond and two single covalent bonds.

Question 4

An element A has 1 electron in its first shell. It combines with element B having 7 electrons in its third shell. What type of bond is formed?

Solution 4

Element A with 1 electron in its first shell is hydrogen, and element B with 7 electrons in its third shell is chlorine.

So, a single covalent bond is formed between hydrogen and chlorine by sharing one pair of electrons.

Question 5

Match the atomic numbers 4,8,10,15 and 19 with each of the following:

1. Element which can form trivalent ion
2. Element with four shells
3. Element with 6 valence electrons
4. Element which does not form ion 

Solution 5

1. Atomic number 15
   Electronic configuration (15): 2,8,5

2. Atomic number 19
   Electronic configuration (19): 2,8,8,1

3. Atomic number 8
   Electronic configuration (8): 2,6

4. Atomic number 10
   Electronic configuration (10): 2,8 

Question 6

If electrons are getting added to en element Y; then

(a) Is Y getting oxidized or reduced?

(b)What charge will Y migrate to during the process of electrolysis?

Solution 6

1. Electrons are getting added to element Y, so, it is getting reduced.
2. Y will migrate towards the positive charge.

Question 7

1. Elements X,Y and Z have atomic numbers 6,9 and 12 respectively. Which one:

 i. Forms an anion

 ii. Forms a cation

2. State the type of bond between Y and Z and give its molecular formula. 

Solution 7

 i. Y = 9

 ii. Z = 12

2. Ionic bond with molecular formula ZY₂. 

Question 8

Taking MgCl₂ as an electrovalent compound, CCl₄ as a covalent compound, give four difference between electrovalent and covalent compounds

Solution 8

Question 9

Potassium chloride is an electrovalent compound, while hydrogen chloride is covalent compound, But, both conduct electricity in their aqueous solutions. Explain.

Solution 9

Potassium chloride is an electrovalent compound and conducts electricity in the molten or aqueous state because the electrostatic forces of attraction weaken in the fused state or in aqueous solution.

Polar covalent compounds like hydrogen chloride ionise in their solutions and can act as an electrolyte. So, both can conduct electricity in their aqueous solutions.

Question 10

Name two compounds that are covalent when taken pure but produce ions when dissolved in water.

Solution 10

HCl and NH₃

Question 11

An element M burns in oxygen to form an ionic compound MO. Write the formula of the compounds formed if this element is made to continue with chlorine and sulphur separately.

Solution 11

Formula of compound when combined with sulphur - MS

Formula of compound when combined with chlorine - MCl₂ 

Question 12

Give orbital diagram of the following:

a) Magnesium chloride

b) Nitrogen

c) Methane

d) Hydrogen chloride

Solution 12

Orbital Diagram:

a) Magnesium chloride

b) Nitrogen

c) Methane

d) Hydrogen chloride

Question 13

State the type of bonding in the following molecules.

1. Water,
2. Calcium oxide
3. Hydrogen chloride 

Solution 13

1. Polar covalent bond
2. Ionic bond
3. Polar covalent bond 

Question 14

Element M forms a chloride with the formula MCl₂ which is a solid with high melting point. What type of bond is in MCl₂ . Write the formula of the compound when M combine with sulphur, oxygen and nitrogen.

Solution 14

The bond formed between metal and nonmetal is ionic bond.

Bond formed between metal M and chlorine is ionic bond.

When metal M combines with sulphur – MgS

When metal M combines with  oxygen – MgO

When metal M combines with nitrogen – Mg₃N₂.

Question 15

1. Mass of an atom is concentrated inside the nucleus of the atom.
2. Atoms combine by transfer and sharing of electron(s).
3. An element has atoms with different mass number.  
4. Carbon-12 and carbon-14 both show similar chemical properties. 

Solution 15

1. Mass of an atom is contributed by mass of protons and mass of neutrons present inside the nucleus of an atom. Electrons present outside the nucleus are of negligible mass. Therefore, the mass of an atom is concentrated inside the nucleus of an atom.
2. Atoms combine because they have incomplete valence shells and tend to attain a stable electronic configuration. So, to attain a stable electronic configuration, atoms of elements gain or share their electrons with other atoms.
3. Mass of an atom is contributed by mass of protons and mass of neutrons present inside the nucleus of an atom. Atoms of the same element contain the same number of protons but may differ in their number of neutrons. Therefore, an element has atoms with different mass number.
4. Carbon-12 and carbon-14 are isotopes of carbon. They have the same atomic number (6) and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. So, carbon-12 and carbon-14 show similar chemical properties.

Question 16

1. The characteristic of an electrovalent compound is that:

a. They are formed by sharing of electrons.

b. They are formed between metals and non-metals.

c. They are formed between two non-metals.

d. They often exist as a liquid.

2. When a metal atom becomes an ion:

a. It loses electrons and is oxidised.

b. It gains electrons and is reduced.

c. It gains electrons and is oxidised.

d. It loses electrons and is reduced. 

Solution 16

(i) B. They are formed between metals and non-metals.

(ii) A. It loses electrons and is oxidised.

Question 17

Identify the following reactions as either oxidation or reduction:

 i. O + 2e^- → O^2-

2. K - e^- → K⁺
3. Fe³⁺ + e^- → Fe²⁺
4. Zn → Zn²⁺ + 2e- 

Solution 17

1. Reduction
2. Oxidation
3. Reduction
4. Oxidation 

Question 18

1. Name the charged particles which attract one another to form electrovalent compounds.
2. In the formation of electrovalent compounds, electrons are transferred from one element to another. How are electrons involved in the formation of a covalent compound?
3. The electronic configuration of nitrogen is (2, 5). How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule?
4. In the formation of magnesium chloride (by direct combination between magnesium and chlorine), name the substance that is oxidized and the substance that is reduced. 

Solution 18

1. Cation and anion
2. By mutual sharing of electrons
3. Two
4. Magnesium is oxidised and chlorine is reduced. 

Question 19

What is the term defined below?

1. A bond formed by a shared pair of electrons, each bonding atom contributing one electron to the pair.
2. A bond formed by transfer of electron(s). 

Solution 19

1. Single covalent bond
2. Electrovalent bond 

Question 20

Name or state the following:

(a) An element having valency zero

(b) Metal with valency one

(c) Atoms of the same element differing in mass number

(d) Elements having same mass number but different atomic number

(e) Bond formed by transfer of electron(s)

(f) Ion formed by gain of electron(s)

Solution 20

(a) Helium

(b) Lithium

(c) Hydrogen: ¹₁H, ²₁H, ³₁H

(d)  ⁴⁰₁₈Ar and ⁴⁰₂₀Ca

(e) Ionic bond

(f) Anion

Question 21

An element X has 2 electrons in its M shell, it forms bond with an element Y which has 7 electrons in its third orbit.

1. Write the formula of the compound formed.
2. Which nearest inert gas electronic configuration will element X and Y acquire.
3. Show by orbital diagram the formation of compound between X and Y. 

Solution 21

Element X has 2 electrons in its M shell.

The electronic configuration would be 2, 8, 2.

So, element X is Mg (12).

Element Y has 7 electrons in its third orbit.

The electronic configuration would be 2, 8, 7.

So, element Y is Cl (17).

1. MgCl₂
2. The nearest inert gas electronic configuration for element X is 2,8, while that for element Y is 2,8,8.
3. Orbital diagram showing the formation of the compound between X and Y:

Magnesium chloride:

Question 22

In the formation of (i) oxygen molecule (ii) carbon tetrachloride molecule, state the following:

1. Electronic configuration of nearest inert gas attained.
2. How many electrons are shared/transferred in bond formation
3. Which type of bonds these compounds form?
4. Draw their orbital diagrams. 

Solution 22

In the formation of

1. Oxygen molecule

1. Neon (10) 2,8
2. Two pairs of electrons are shared.
3. Covalent bond
4. Orbital Diagram: