The Gas composition of 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.Nitrogen 78.084Oxygen 20.9476Argon Ar 0.934Carbon Dioxide 0.0314
To give a familiar example, air has a composition of:[1]
| Pure Gas Name | Symbol | Percent by Volume | |
|---|---|---|---|
| Nitrogen | N2 | 78.084 | |
| Oxygen | O2 | 20.9476 | |
| Argon | Ar | 0.934 | |
| Carbon Dioxide | CO2 | 0.0314 | |
| Neon | Ne | 0.001818 | |
| Methane | CH4 | 0.0002 | |
| Helium | He | 0.000524 | |
| Krypton | Kr | 0.000114 | |
| Hydrogen | H2 | 0.00005 | |
| Xenon | Xe | 0.0000087 |
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: