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
(By definition, the molar mass of a gas is the ratio of the mass of one molecule of gas to that of an hydrogen atom under similar conditions.)
Therefore:
vapour density = molar mass of gas / molar mass of H2
vapour density = molar mass of gas / 2
vapour density = ½ × molar mass
(and thus: molar mass = 2 × vapour density)
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 vapor density of one. For this use, Air has a molecular weight of 29 atomic mass units, and all other gas and vapor molecular weights are divided by this number to derive their vapor density.[2] For example, Acetone has a vapor density of 2[3] in relation to air. That means Acetone vapor is twice as heavy as air. This can be seen by dividing the molecular weight of Acetone, 58.1[4] by that of air, 29, 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 a smothering hazard to individuals entering the lower part of that space.
It is recommended that individuals confirm the definition of vapour density being used in their specific context.
See also
- relative density (also known as specific gravity)