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Vapour density is a measure of the density of a gas or vapour in comparison to the density of hydrogen at the same temperature and pressure. It is calculated as the mass of a certain volume of a substance divided by the mass of the same volume of hydrogen. Materials lighter than air have vapour densities less than 1, while those heavier than air have vapour densities greater than 1. The formula for calculating vapour density is molar mass of gas divided by molar mass of hydrogen, or approximately twice the vapour density. Vapour density can be expressed in units of grams or kilograms per cubic meter.
