Vapour density
relative density used for gases
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 gas .
vapour density = molar mass of gas / molar mass of H2
vapour density = molar mass of gas / 2.01568
vapour density = 1⁄2 × 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 has a vapour density of 2 in relation to air. That means acetone vapour is twice as heavy as air.
The public source identifies “Vapour density” as relative density used for gases. This brief keeps that definition visible, then builds a research path around Vapour, density and relative.
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This entry incorporates text from “Vapour density” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.