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.

vapour density = mass of n molecules of gas / mass of n molecules of hydrogen


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

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.[1] 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.[2] For example, acetone has a vapour density of 2[3] 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[4] by that of air, 28.97, which equals 2.

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.

See also


  1. MSDS Glossary of Terms – Vapor Density. Retrieved on 2012-02-09.
  2. HazMat Math: Calculating Vapor Density. Retrieved on 2012-02-09.
  3. MSDS: Acetone. (1998-04-21). Retrieved on 2012-02-09.
  4. NIOSH Pocket Guide: Acetone. Retrieved on 2012-02-09.
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