relation between Molecular wt. and vapour density.
Vapour density of a gas = Density of the gas / density of hydrogen
= Mass of certain volume of the gas at STP / mass of the same volume of H2 at STP
= Mass of n molecules of the gas / Mass of n molecules of H2
= Mass of 1 molecule of the gas / Mass of 1 molecule (or 2 atoms of H)
= Molecular mass / 2
Thus we get
Molecular mass = 2 X vapour density