Gas composition

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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.

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

Pure Gas Name	Symbol	Mole Fraction
Nitrogen	N2	0.78084
Oxygen	O2	0.209476
Argon	Ar	0.00934
Carbon Dioxide	CO2	0.000314
Neon	Ne	0.00001818
Methane	CH4	0.000002
Helium	He	0.00000524
Krypton	Kr	0.00000114
Hydrogen	H2	0.0000005
Xenon	Xe	0.000000087

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.

[edit] 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.