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.
vapour density = molar mass of gas / molar mass of H2
vapour density = molar mass of gas / 2.016
vapour density = ~½ × molar mass