What is Vapor Density?

Vapour density is the density of a vapour in relation to that of hydrogen. 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 n molecules of hydrogen

Therefore:

vapour density = molar mass of gas / molar mass of H2

vapour density = molar mass of gas / 2.016

vapour density = ~½ × molar mass

(and thus: molar mass = ~2 × vapour density)