Avogadro’s Hypothesis

​Berzelius a Swedish chemist, gave a hypothesis called Berzelius hypothesis which states that:

Equal volume of all gases under similar conditions of temperature and pressure contain equal number of atoms.

For Example: Hydrogen + Chlorine → 2 Hydrogen chloride gas

1 vol                1 vol                                     2 vol

n atoms            n atoms                             2n molecule

On dividing throughout by 2n

½ atom              ½ atom                             n molecules

This implies that one compound atom of hydrogen chloride is made up of ½ atom of hydrogen and ½ atom of chlorine. This is in direct conflict with Dalton’s atomic theory which states that atoms are the ultimate particles of elements and are indivisible. This hypothesis was therefore rejected.

Avogadro’s hypothesis

It states that equal volume of all gases under similar condition of temperature and pressure contain equal number of molecules .

Applications of Avogadro’s Law

1) In the calculation of atomicity of Elementary gases

Atomicity of an elementary substance is defined as the number of atoms of the element present in 1 molecule of the substance.

For example: Atomicity of oxygen is 2 while that of ozone is 3.

Hydrogen       +       oxygen → Water Vapours

2n molecules             n molecules                   2n molecules

On dividing throughout by 2n

1 molecule                  ½ molecule                       1 molecule

Thus one molecule of water contains ½ molecule of oxygen. But 1 molecule of water contains 1 atom of oxygen.

Hence ½ molecule = 1 molecule of oxygen

1 molecule of oxygen= 2 atoms of oxygen = 1 atom of oxygen= 2

2) To find the relationship between molecular mass and vapour density of a gas

Molecular mass = 2 × vapour density

3) To find the relationship between mass and volume of a gas

22.4 litres of any gas at STP weigh equal to the molecular mass of the gas expressed in grams. This is called gram molecular volume law.