Vapor 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. vapor density = mass of n molecules of gas / mass of n molecules of hydrogen.Therefore:vapour density = molar mass of gas / molar mass of H 2.or example, acetone has a vapour density of 2 in relation to air. That means acetone vapour is twice as heavy as air. This can be seen by dividing the molecular weight of Acetone, 58.1 by that of air, 28.97, which equals 2.