Question: what is the relation between molecular mass and vapour density of gas?
Answer: It may be defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen. vapour density= mass of n molecules of gas / mass of nmolecules of hydrogen. (and thus: molar mass = ~2 × vapour density) For example,vapour density of mixture of NO2 and N2O4 is 38.