In 1811 Italian scientist Amedeo Avogadro tried to combine conclusions of Dalton’s atomic theory and Gay Lussac’s law of combining volumes (Unit 1) which is now known as Avogadro law.

It states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.

This means that as long as the temperature and pressure remain constant, the volume depends upon number of molecules of the gas or in other words amount of the gas.

The number of molecules in one mole of a gas has been determined to be 6.022 ×10^{23} and is known as Avogadro constant. You will find that this is the same number which we came across while discussing definition of a ‘mole’ (Unit 1).

Since volume of a gas is directly proportional to the number of moles; one mole of each gas at standard temperature and pressure (STP)* will have same volume.

Standard temperature and pressure means 273.15 K (0°C) temperature and 1 bar (i.e., exactly 10 ^{5} pascal) pressure. These values approximate freezing temperature of water and atmospheric pressure at sea level.

At STP molar volume of an ideal gas or a combination of ideal gases is 22.71098 L mol^{–1}.

Number of moles of a gas can be calculated as follows n = m/M

Where m = mass of the gas under investigation and M = molar mass. Thus, V= k4 (m/M).

Equation can be rearranged as follows : M = k4 d.

Here ‘*d*’ is the density of the gas. We can conclude from equation that the density of a gas is directly proportional to its molar mass.

A gas that follows Boyle’s law, Charles’ law and Avogadro law strictly is called an ideal gas. Such a gas is hypothetical. It is assumed that intermolecular forces are not present between the molecules of an ideal gas.

Real gases follow these laws only under certain specific conditions when forces of interaction are practically negligible. In all other situations these deviate from ideal behaviour.