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.
vapour density = mass of n molecules of gas / mass of n molecules of
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) For example, vapour
density of mixture of NO2 and N2O4 is 38. 3.
Vapour density is a
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
With this definition, the vapour density would indicate whether a gas
is denser (greater than one) or less dense (less than one) than air.
The density has implications for container storage and personnel
safety—if a container can release a dense gas, its vapour could sink
and, if flammable, collect until it is at a concentration sufficient
for ignition. Even if not flammable, it could collect in the lower
floor or level of a confined space and displace air, possibly
presenting an asphyxiation hazard to individuals entering the lower
part of that space.
Relative density (also known as specific gravity)
Victor Meyer apparatus
^ MSDS Glossary of Terms – Vapor Density. Msdsonline.com. Retrieved
^ HazMat Math: Calculating Vapor Density. Firenuggets.com. Retrieved
^ MSDS: Acetone. Hazard.com (1998-04-21). Retrieved on 2012-02-09.
^ NIOSH Pocket Guide: Acetone. Cdc.gov. Retrieved on