1. ChemistryVapour 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 gas . vapour density = molar mass of gas / molar mass of H2 vapour density = molar mass of gas / 2.01568 vapour density = 1⁄2 × molar mass (and thus: molar mass = ~2 × vapour density) For example, vapour density of mixture of NO2 and N2O4 is 38.3. Vapour density is a dimensionless quantity. Vapour density = density of gas / density of hydrogen (H2) == 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... from wikipedia.org