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.
Standard Dry Air -- an agreed-upon gas composition for air from which all water vapour has been removed. There are various standards bodies which publish documents that define a dry air gas composition. Each standard provides a list of constituent concentrations, a gas density at standard conditions and a molar mass.
It is extremely unlikely that the actual composition of any specific sample of air will completely agree with any definition for standard dry air. While the various definitions for standard dry air all attempt to provide realistic information about the constituents of air, the definitions are important in and of themselves because they establish a standard which can be cited in legal contracts and publications documenting measurement calculation methodologies or equations of state.
The standards below are two examples of commonly used and cited publications that provide a composition for standard dry air.
[1]ISO 6976:1995 provides a definition for standard dry air which specifies an air molar mass of 28.9626 g/mol.
[2]GPA 2145:2009 is published by the Gas Processors Association. It provides a molar mass for air of 28.9625 g/mol, and provides a composition for standard dry air as a footnote.