Gaseous State

Question 1

What is a Gas?

Answer 1

A gas consists of molecules separated wide apart in empty space. The molecules are free to move about throughout the container.

Question 2

What is Liquid?

Answer 2

A liquid has molecules touching each other. However, the intermolecular space, permit the movement of molecules throughout the liquid.

Question 3

What is a Solid?

Answer 3

A solid has molecules, atoms or ions arranged in a certain order in fixed positions in the crystal lattice. The particles in a solid are not free to move about but vibrate in their fixed positions.

Question 4

GENERAL CHARACTERISTICS OF GASES

Answer 4

1. Expansibility: Gases have limitless expansibility. They expand to fill the entire vessel they are placed in.
2. Compressibility: Gases are easily compressed by applying pressure to a movable piston fitted in the container.
3. Diffusibility: Gases can diffuse rapidly through each other to form a homogeneous mixture.
4. Pressure: Gases exert pressure on the walls of the container in all directions.
5. Effect of Heat: When a gas, confined in a vessel is heated, its pressure increases. Upon heating in a vessel fitted with a piston, the volume of the gas increases.

The above properties of gases can be easily explained by the Kinetic Molecular Theory.

Question 5

Parameters of a gas

Answer 5

1. The volume, V of the gas
2. Its pressure, P
3. its temperature, T
4. the number of moles, n, of the gas in the container

Question 6

What is the volume of a gas?

Answer 6

The volume of the container is the volume of the gas sample.
1 Liter = 1000 mL
1 mL = 1 cc (cm3)
1 L = 1000.028 cc
The SI unit of volume is Cubic Meter (m3).

Question 7

What is the Pressure of a Gas?

Answer 7

The pressure of a gas is defined as the force exerted by the impacts of its molecules per unit surface area in contact. The pressure of a gas sample can be measured with the help of a mercury manometer.

Question 8

Definition of 1 atm

Answer 8

The pressure of air that can support 760 mm Hg column at sea level, is called one atmosphere.
1 atm = 760 mm Hg = 760 torr
The SI unit of pressure is the Pascal (Pa).
1 atm = 760 torr = 1.013 × 105 Pa

Question 9

What is the Temperature of a Gas?

Answer 9

The temperature of a gas may be measured in Centigrade degrees (°C) or Celsius degrees. The SI unit of temperature is Kelvin (K) or Absolute degree.
The Kelvin temperature (or absolute temperature) is always used in calculations of other parameters of gases.

Question 10

What is Gas Laws?

Answer 10

The relationships between the volume, temperature and pressure of given mass of gas sample are called the gas laws.

Question 11

Boyle’s Law

Answer 11

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure. If the pressure is doubled, the volume is halved.

Question 12

Charles Law

Answer 12

At constant pressure, the volume of a fixed mass of gas is directly proportional to the Kelvin temperature of absolute temperature. If the absolute temperature is doubled, the volume is doubled.

Question 13

The Combined Gas Law,

Answer 13

For a fixed mass of gas, the volume is directly proportional to kelvin temperature and inversely proportional to the pressure.

Question 14

Gay Lussac’s Law

Answer 14

At constant volume, the pressure of a fixed mass of gas is directly proportional to the Kelvin temperature or absolute temperature. It’s also known as the Pressure- Temperature Law of gas.

Question 15

Avogadro’s Law,

Answer 15

Equal volumes of gases at the same temperature and pressure contain equal number of moles or molecules. If the molar amount is doubled, the volume is doubled.

Question 16

The Ideal Gas Equation

Answer 16

The volume of a given amount of gas is directly proportional to the number of moles of gas, directly proportional to the temperature, and inversely proportional to the pressure.

Question 17

Why the ideal-gas equation is called an Equation of State?

Answer 17

The ideal-gas equation is called an Equation of State for a gas because it contains all the variables (T, P, V and n) which describe completely the condition or state of any gas sample. If we know the three of these variables, it is enough to specify the system completely because the fourth variable can be calculated from the ideal-gas equation.

Question 18

The Numerical Value of R is,

Answer 18

• 0.0821 atm. liter K–1 mol–1
• 82.1 ml-atm K–1 mol–1
• 62.3 liter-mm Hg K–1 mol–1
• 8.314 × 107 erg K–1 mol–1
• 8.314 Joule K–1 mol–1
• 1.987 Cal K–1 mol–1

Question 19

Partial Pressure

Answer 19

In a mixture of gases, each component gas exerted a pressure as if it were alone in the container. The individual pressure of each gas in the mixture is called the Partial Pressure of that gas.

Question 20

Dalton’s Law of Partial Pressures,

Answer 20

The total pressure of a mixture of gases is equal to the sum of the partial pressures of all the gases present.

Question 21

Diffusion:

Answer 21

When two gases are placed in contact, they mix spontaneously. This is due to the movement of molecules of one gas into the other gas. This process of mixing of gases by random motion of the molecules is called Diffusion.

Question 22

Graham’s Law of Diffusion

Answer 22

Under the same conditions of temperature and pressure, the rates of diffusion of different gases are inversely proportional to the square roots of their molecular masses.

Question 23

Graham’s Law of Effusion.

Answer 23

Graham’s law when applied to effusion of a gas is called the Graham’s Law of Effusion.

Question 24

Why Graham’s law of effusion is often used to find the molecular mass of a given gas?

Answer 24

The determination of rate of effusion is much easier compared to the rate of diffusion. Therefore, Graham’s law of effusion is often used to find the molecular mass of a given gas.

Question 25

What is Kinetic Molecular Theory?

Answer 25

In 1859, Maxwell and Boltzmann developed a mathematical theory to explain the behaviours of gases and gas laws. It is based on the fundamental concept that a gas

is made of a large number of molecules in perpetual motion. Hence the theory is called the kinetic molecular theory or simply the kinetic theory of gases.

Question 26

Assumptions of the Kinetic Molecular Theory

Answer 26

1. A gas consists of extremely small discrete particles called molecules dispersed throughout the container. The actual volume of the molecules is negligible compared to the total volume of the gas. The molecules of a given gas are identical and have the same mass (m).
2. Gas molecules are in constant random motion with high velocities. They move in straight lines with uniform velocity and change direction on collision with other molecules or the walls of the container.
3. The distance between the molecules is very large and it is assumed that van der Waals attractive forces between them do not exist. Thus, the gas molecules can move freely, independent of each other.
4. All collisions are perfectly elastic. Hence, there is no loss of the kinetic energy of a molecule during a collision.
5. The pressure of a gas is caused by the hits recorded by molecules on the walls of the container.

The average kinetic energy (1/2 mv^2) of the gas molecules is directly proportional to absolute temperature (Kelvin temperature). This implies that the average kinetic energy of molecules is the same at a given temperature.

Question 27

Whai is an Ideal Gas?

Answer 27

A gas that confirms the assumptions of the kinetic theory of gases is called an ideal gas. It obeys the basic laws strictly under all conditions of temperature and pressure.

Question 28

How Does an Ideal Gas Differ from Real Gases?

Answer 28

The real gases such as hydrogen, oxygen, nitrogen etc., are opposed to the assumptions of the Kinetic Molecular Theory. Thus:
a) The actual volume of molecules in an ideal gas is negligible, while in a real gas it is appreciable.
b) There are no attractive forces between molecules in an ideal gas while these exist in a real gas.
c) Molecular collisions in an ideal gas are perfectly elastic while it is not so in a real gas.
For the reasons listed above, real gases obey the gas laws under moderate conditions of temperature and pressure. At very low temperatures and very high-pressure real gases show considerable deviations from the ideal gas behaviours.

Question 29

When a real gas obeys the assumptions of the Kinetic Molecular theory?

Answer 29

At very low temperature and very high-pressure real gases obeys the assumptions of the Kinetic Molecular Theory.