Gas composition


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.

Gas composition of air

To give a familiar example, air has a composition of:
Pure Gas NameSymbolPercent by Volume
NitrogenN278.084
OxygenO220.9476
ArgonAr0.934
Carbon DioxideCO20.0314
NeonNe0.001818
MethaneCH40.0002
HeliumHe0.000524
KryptonKr0.000114
HydrogenH20.00005
XenonXe0.0000087
Standard Dry Air is the 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: