Vapour density is the

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

of a

vapour In physics, a vapor (American English) or vapour (Commonwealth English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Herring, ''General ...

in relation to that of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

. 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 gas . :vapour density = molar mass of gas / molar mass of H₂ :vapour density = molar mass of gas / 2.01568 :vapour density = × 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 dimensionless quantity. Vapour density = density of gas / density of hydrogen (H2)

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. 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 Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

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, 58.1NIOSH Pocket Guide: Acetone
Cdc.gov. Retrieved on 2012-02-09. 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.

References

Density Gases {{Fluiddynamics-stub

Alternative definition

See also

References