Avogadro Hypothesis

AVOGADRO HYPOTHESIS

In 1811 Avogadro framed a hypothesis based on the relationship between the ­number of molecules present in equal volumes of gases in different conditions­.

The Avogadro’s law states that “equal volumes­ of all gases under similar conditions of temperature and pressure contain equal number of molecules”

It follows that the volume of any given gas must be proportional to the number of molecules in it. If ‘V’ is the volume and ‘n’ is the number of molecules of a gas, then Avogadro law is represented, mathematically, as follows:

V α n

V = constant × n

Thus, one litre (1 dm³) of hydrogen contains the same number of molecules as in one litre of oxygen, i.e. the volume of the gas is directly proportional to the number of molecules of the gas.

Explanation

Let us consider the reaction between ­hydrogen and chlorine to form hydrogen chloride gas

H_2(g) + Cl_2(g) → 2 HCl_(g)

1 vol + 1 vol → 2 volumes

According to Avogadro’s law 1 volume of any gas is occupied by “n” number of molecules. n molecules + n molecules → 2n molecules

if n = 1 then

1molecule + 1 molecule → 2 molecules.

molecule + ½ molecule → 1 molecule

1 molecule of hydrogen chloride gas is made up of ½ molecule of hydrogen and ½ molecule of chlorine. Hence, the molecules can be subdivided. This law is in agreement with Dalton’s atomic theory.