A gas cylinder of capacity of 20 dm3 is filled with gas X the mass of which is 10 g. When the same cylinder is filled with hydrogen gas at the same temperature and pressure the mass of the hydrogen is 2 g, hence the relative molecular mass of the gas is:
10
Vapour density of gas = Mass of gas/Mass of an equal volume of hydrogen at the same temp. and press
= 10/2 = 5
Relative Molecular mass = 2 x Vapour density
= 2 x 5
= 10