THE GAS LAWS

• Boyle’s Law (Pressure-Volume Relationship)
At constant temperature, the volume of a given mass of gas is inversely proportional to its pressure. It states that, under isothermal conditions pressure of a given mass of a gas is inversely proportional to its volume.

According to this the volume of a given mass of gas is inversely proportional to pressure at constant temperature. This law is given by Robert Boyle.

P 1  …… (at constant T and n)

 V

P = k 1

V

Where k is the proportionality constant.

If a fixed amount of gas at constant temperature T occupying volume V1 at pressure P1 undergoes expansion so that volume becomes V2 and pressure becomes P2, then according to Boyle’s law.

P1V1 = P2V2 = constant

 P1 = V2

P2    V1

• Charles’ law: 

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

V T

or, V = constant

T

Let V1 be the volume of a gas at temperature T1. Pressure remains constant.

If the temperature of the gas increased to T2, then the volume will also increase to V2.

Therefore, according to law,

V1 = V2

T1    T2

• Gay Lussac’s Law (Pressure-Temperature Relationship)
At constant volume, pressure of a given mass of a gas is directly proportional to the temperature.

Mathematically,

P T

      P = k (at constant volume, and mass)

T

Where, k is the proportionality constant.

If a fixed amount of gas at constant volume V occupying pressure P1 at temperature T1 undergoes expansion, so that pressure becomes P2 and temperature becomes T2 then according to lussac’s law

P = constant

T

P1P2

T1     T2

• Avogadro Law (Volume-Amount Relationship)
Avogadro’s law states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules. V α n

Where n is the number of moles of the gas.
Avogadro constant: The number of molecules in one mole of a gas
= 6.022 x 1023

Ideal Gas: A gas that follows Boyle’s law, Charles’ law and Avogadro law strictly, is called an ideal gas.
Real gases follow these laws only under certain specific conditions. When forces of interaction are practically negligible.