What is "vapour density"

n. (context physics English) The ratio of the density of a gas to the density of hydrogen at the same temperature and pressure.

n. the density of a gas relative to the density of hydrogen [syn: 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.

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

vapour density = molar mass of gas / 2.016

vapour density = ~½ × molar mass

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