Molecular weight: It is the ratio of the weight of 1 molecule of a substance to the weight of one atom of hydrogen.
Mol.wt = mass of 1 molecule of a substance/mass of 1 mol. of hydrogen
Vapour Density : It is the ratio of the mass of a certain volume of gas or vapour of the mass of the same volume of hydrogen.
Mol. wt. = 2 x vapour density