Vapour 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.
vapor 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)
Alternative definition
In many web sources, particularly in relation to safety considerations at commercial and industrial facilities in the U.S., vapour density is defined with respect to air, not hydrogen. Air is given a vapour density of one. For this use, air has a molecular weight of 28.97 atomic mass units, and all other gas and vapour molecular weights are divided by this number to derive their vapour density. For 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.
