Gas composition

Gas composition: any gas can be characterised by listing the pure substances it contains, and stating for each substance its proportion of the gas mixture's molecule count.

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To give a familiar example, air has a composition available here, from CRC Handbook of Chemistry and Physics, 1997 Edition of:

The molar fractions of a complete gas composition must add up to 1.0000.

Note: a gas composition is sometimes also represented in mole percentages. These are simply a factor of 100 larger than molar fractions.

See also

Standard Dry Air -- an agreed upon gas composition for air, the most important point of which to note is that its mixture molar mass is 28.9625. You can contribute to this article by specifying what standards body defines standard dry air, and the name of the standard.