Vapour density is the
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of a
vapour
In physics, a vapor (American English) or vapour (British English and Canadian 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. Herr ...
in relation to that of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
. 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 H
2
:vapour density = molar mass of gas / 2.016
:vapour density = × molar mass
(and thus: molar mass = ~2 × vapour density)
For example, vapour density of mixture of NO
2 and N
2O
4 is 38.3. Vapour density is a dimensionless quantity.
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 formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscib ...
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
NIOSH 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.
See also
*Relative density
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
(also known as specific gravity)
*Victor Meyer apparatus
The Victor Meyer apparatus is the standard laboratory method for determining the molecular weight of a volatile liquid. It was developed by Viktor Meyer, who spelled his name ''Victor'' in publications at the time of its development. In this met ...
References
Density
Gases
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