Chapter 2 – Behaviour of Gases

Very Short Answer Type Question

Question 1

What is torr?

Answer

Torr is a unit of measurement for pressure named after the Italian physicist Evangelista Torricelli, who invented the mercury barometer in 1643.

Question 2

Which state of matter has no definite volume and no definite shape?

Answer

Gas.

Question 3

What is the value of the gas constant in SI units?

Answer

The value of the gas constant in the SI unit is 8.314 J/K/mol.

Question 4

How is unit ‘L’ related to dm³ ?

Answer

One litre is equal to one cubic decimeter, which can be expressed as:

1 L = 1 dm³

Question 5

Name the SI unit of pressure.

Answer

Pascal

Question 6

What is the value of temperature absolute zero on the celsius scale?

Answer

Absolute zero is equal to -273.15 °C.

Question 7

What is meant by STP ?

Answer

STP stands for “standard temperature and pressure”.

• Standard temperature: 0°C or 273.15 K
• Standard pressure: 1 atmosphere (atm) or 101.325 kilopascals (kPa) or 760 torr

Question 8

Name the law governing the expansion of ideal gases at constant Pressure.

Answer

Charles’s Law.

Question 9

Under what conditions is Boyles’s law applicable?

Answer

Boyles’s law is applicable only when

• Temperature is constant
• Mass is constant
• Gas is ideal

Question 10

What is the effect of increasing temperature on the vapour of a liquid at constant pressure?

Answer

When the temperature of a liquid is increased at constant pressure, the vapour pressure of the liquid also increases.

Question 11

Write the value of R in L atm mol^-1K^-1.

Answer

0.0821 L atm/mol/k.

Question 12

Express Boyle’s Law and Charle’s law in the form of an equation.

Answer

Boyle’s law :

Pressure × Volume = K

Or, PV = K

Charle’s law :

V_t = V_o(1+t/273)

Question 13

Out of dry air and wet air which is heavier?

Answer

Dry air is heavier than moist air.

Question 14

What is the cause of gas pressure?

Answer

Gas pressure is caused by the constant and random motion of gas molecules, which collide with each other and with the walls of the container they are in.

Question 15

The expandability of a gaseous substance is very high – why?

Answer

The expandability or compressibility of a gaseous substance is very high because gases have a lot of empty space between their molecules.

Question 16

What are the variables and constant quantities in Gay Lussac’s law?

Answer

The variables in Gay-Lussac’s Law are pressure (P) and temperature (T), while the constant quantity is the volume (V).

Question 17

What are the constants of Boyle’s Law?

Answer

Mass and temperature of the gas remains constant.

Short Answer Type Question

Question 18

What is the normal temperature and Pressure?

Answer

Normal temperature and pressure (NTP) is a reference condition used for defining the standard temperature and pressure (STP) of gases.

The normal temperature is defined as 0°C (273.15 K), while the normal pressure is defined as 1 atmosphere (atm) or 101.325 kilopascals (kPa) or 760 torrs.

Question 19

Why is Boyle’s law not applicable when a balloon is blown with air?

Answer

Boyle’s law is not applicable when a balloon is blown with air because

1. The amount of gas is not constant
2. The temperature is not constant

Question 20

State Boyle’s Law.

Answer

At constant temperature, the volume of a given mass of a gas is inversely proportional to its pressure.

i.e  At constant temperature and mass,

V $\propto$ 1/P (Where V and P are the volume and pressure of the gas respectively)

or, PV = constant

Question 21

State Charle’s Law?

Answer

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

Question 22

Write down the relationship between the Celcius scale of temperature and the Absolute scale of temperature.

Answer

If a temperature on Kelvin scale in T K and the temperature on Celcius scale is t^oC, the relation is :

T = t + 273

Question 23

What is an ideal gas?

Answer

An ideal gas is a theoretical gas composed of a large number of molecules that are assumed to have no volume and no intermolecular forces between them, except for perfectly elastic collisions.

Question 24

State Charle’s law in terms of Absolute temperature.

Answer

At constant pressure, the volume (V) of a given mass of a gas is directly proportional to its absolute temperature (T). i.e V ∝ T (at constant pressure).

Question 25

What is the universal gas constant?

Answer

The universal gas constant is a physical constant denoted by the symbol R. It relates the properties of an ideal gas to each other and is the same for all ideal gases. The value of the universal gas constant depends on the units used to express the other properties of the gas.

Question 26

Define gram molecular volume.

Answer

The volume occupied by one gram molecule of any gaseous substance at a fixed temperature and pressure is known as the gram molecular volume.

Question 27

What is Avogadro’s number?

Answer

Avogadro’s number is a fundamental constant denoted by the symbol “NA”. It is defined as the number of constituent particles, such as atoms, molecules, or ions, in one mole of a substance. The value of Avogadro’s number is approximately 6.022 x 10²³ particles per mole.

Question 28

What is Avogadro’s Law?

Answer

Avogadro’s Law is a gas law that states that, under the same temperature and pressure conditions, equal volumes of different gases contain an equal number of particles, such as atoms or molecules.

Long Answer Type Question

Question 29

State the characteristics of gases.

Answer

Gases have the following characteristics:

1. Compressibility: Gases are highly compressible because the molecules in gases are far apart from each other, and their intermolecular forces are weak. Therefore, when pressure is applied, gases can be compressed to occupy smaller volumes.
2. Expansibility: Gases are highly expansible because their molecules move randomly in all directions, and there is no fixed shape or volume for gases.
3. Low Density: Gases have low density because their molecules are widely spaced apart from each other, and they occupy a large volume relative to their mass.
4. High Diffusivity: Gases have high diffusivity because of their random motion, and they can easily spread out and mix with other gases.

Question 30

State Boyle’s law and explain it.

Answer

Boyle’s Law states that, for a fixed amount of gas at a constant temperature, the volume of the gas is inversely proportional to its pressure. Mathematically, this law can be expressed as:

PV = constant

where P is the pressure of the gas, V is its volume, and the product of P and V is a constant at a given temperature.

Question 31

State Charle’s law and explain it.

Answer

Charles’s Law states that, for a fixed amount of gas at a constant pressure, the volume of the gas is directly proportional to its absolute temperature. Mathematically, this law can be expressed as:

V/T = constant

where V is the volume of the gas, T is its absolute temperature (in Kelvin), and the ratio of V and T is a constant at a given pressure.

Question 32

What is the absolute zero temperature? Define the absolute scale of temperature.

Answer

Absolute zero is the lowest possible temperature, at which the particles of matter have minimal kinetic energy. The absolute scale of temperature is a temperature scale that starts at absolute zero, where the temperature is measured in kelvin (K), and zero kelvin corresponds to -273.15 °C on the Celsius scale.

Question 33

How is the Absolute zero obtained from Charles’ law?

Answer

From Charle’s law, we know that, at constant pressure, the volume of a given mass of gas increases or decreased by $\frac{\mathrm{t/}}{}$ part of its volume at 0^oC for every 1 degree rise or fall in temperature.

Suppose, the temperature of the same gas is decreased to -t^oC. So, its volume at that temperature,

V_t = V_o(1+t/273)

From the above equation, it is evident that if the temperature is reduced to -273^oC, V_t will be zero.

Based on this observation, a new scale of temperature was adopted with its zero at -273^oC. This scale is known as the absolute scale and the zero reading of this scale is known as absolute zero.

Question 34

Establish the combined law of Boyle’s and Charle’s law.

Answer

Let V be the volume of a given mass of gas at pressure P and temperature T (absolute),

From Boyle’s law,

V ∝ 1/P (m and T constant)

From Charle’s law,

V ∝ T (m and P constant)

By combining two laws we have,

V ∝ T/P or,

V = K T/P (Where K = proportionality constant)

or, PV = KT

or, PV = nRT

Where, n is the number of moles of gas, R is the universal gas constant (0.08206 L·atm/mol·K)

Question 35

What are the basic assumptions of the kinetic theory of gases?

Answer

The basic consumption of the kinetic theory of gases are :

1. A gas consists of a collection of a very large number of minute discrete particles called molecules, they are spherical, perfectly elastic particles.
2. The molecule of a gas within a container is in chaotic motion in all directions with high velocities continually colliding with each other and with the wall of the container.
3. The average kinetic energy of the gas molecules in the system measures the temperature of the gas.
4. The volume of the gaseous molecule is considered negligible related to the total volume of the gas.

Objective Type Question

Fill in the blanks

1. An equal volume of two gases under similar conditions of temperature contains ____ number of molecules.
2. The volume of a gas is zero at ____ temperature.
3. At constant temperature for a given volume of a gas the product of its pressure and its volume is ____
4. The velocities of molecules ______ with rise of temperature.
5. The equation of ideal gas for n gram-moles is _____
6. The value of absolute zero is ____ on the celsius scale.

Answer

1. Equal
2. absolute
3. constant
4. increase
5. PV=nRT
6. – 273^oC

State Whether True or False?

1. All gases known so far are ideal gases.
2. No deviation from Avogadro’s law is observed in the case of real gases.
3. The universal gas constant depends upon the nature of the gases.
4. The volume of gas always decreases if its pressure is decreased.
5. The value of Avogadro’s number is 6.022 × 10²³

Answer

1. False
2. False
3. False
4. False
5. True

Match the following

Answer

1. (d)
2. (c)
3. (b)
4. (e)
5. (a)

Choose the correct answer

(i) Volume of 4.4 g of CO₂ at STP is –

1. 22.4 L
2. 2.24 L
3. 224 L
4. 44.8 L

Answer

(b) 2.24 L

Explanation:

Molar mass of CO₂ = 12 + 16 × 2 = 44 g

Number of moles (n) = 4.4/44 = 0.1

Temperature (T) = 273 K

Pressure (P) = 1 atm

Gas constant (R) = 0.0821 L.atm/mol.K

PV = nRT

(1 atm) × V = (0.1 moles) × (0.0821 L.atm/mol.K) × (273 K)

V = 0.1  × 0.0821 × 273 /1 = 2.24 L

(ii) Gas deviates from ideal gas nature because molecules

1. are colourless
2. attract each other
3. contain covalent bond
4. show Brownian movement

Answer

(b) attract each other

(iii) The number of gram molecules of oxygen is 6.02 × 10²⁴ Co molecule is –

1. 8 g molecules
2. 5 g molecules
3. 2 g molecules
4. 0.5 g molecules

Answer

(b) 5 g molecules

Explanation

Number of moles = Number of molecules / Avogadro’s number

Number of moles of CO = 6.02 × 10²⁴ / 6.02 × 10²³ = 10

Number of moles of O2 = 0.5 × Number of moles of oxygen atoms

Number of moles of O2 = 0.5 × 10 moles = 5 moles

So, 6.02 × 10^24 CO molecules is equivalent to 5 gram molecules of oxygen.

(iv) A gas will approach ideal behaviours at –

1. at low temperatures and low pressure
2. low temperature and high pressure
3. high temperature and low temperature
4. high temperature and high pressure

Answer

(c) high temperature and low temperature

(v) Kinetic theory of gases proves

1. only Boyle’s law
2. only Charles law
3. only Avogadro’s law
4. all of these

Answer

(d) all of these

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