# Gas composition

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{{Missing information|the actual subject|date=April 2018}}
The '''gas composition''' of any [gas](/source/gas) can be characterised by listing the [pure substance](/source/pure_substance)s it contains, and stating for each substance its proportion of the gas mixture's [molecule](/source/molecule) count. Nitrogen	{{chem2|N2}}	78.084
Oxygen	{{chem2|O2}}	20.9476
Argon	Ar	0.934
Carbon Dioxide	{{CO2}}	0.0314

==  Gas composition of air ==
To give a familiar example, [air](/source/air) has a composition of:<ref>{{Cite web|url=http://www.physlink.com/Reference/AirComposition.cfm|title=Air Composition at Sea-Level|last=Skorucak|first=PhysLink.com, Anton|website=www.physlink.com|language=en|access-date=2018-04-20}}</ref>
{| class="wikitable"
|-
! Pure Gas Name
! [Symbol](/source/List_of_elements_by_symbol)
! Percent by Volume
|-
|[Nitrogen](/source/Nitrogen)
|N<sub>2</sub>
|78.084
|-
|[Oxygen](/source/Oxygen)
|O<sub>2</sub>
|20.9476
|-
|[Argon](/source/Argon)
|Ar
|0.934
|-
|[Carbon Dioxide](/source/Carbon_Dioxide)
|CO<sub>2</sub>
|0.0314
|-
|[Neon](/source/Neon)
|Ne
|0.001818
|-
|[Methane](/source/Methane)
|CH<sub>4</sub>
|0.0002
|-
|[Helium](/source/Helium)
|He
|0.000524
|-
|[Krypton](/source/Krypton)
|Kr
|0.000114
|-
|[Hydrogen](/source/Hydrogen)
|H<sub>2</sub>
|0.00005
|-
|[Xenon](/source/Xenon)
|Xe
|0.0000087
|}[Standard Dry Air](/source/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](/source/gas_density) at [standard conditions](/source/standard_conditions) and a [molar mass](/source/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](/source/equations_of_state).

The standards below are two examples of commonly used and cited publications that provide a composition for standard dry air:

* ISO TR  29922-2017 provides a definition for standard dry air which specifies an air molar mass of 28,965 46 ± 0,000 17&nbsp;kg·kmol-1.<ref name="ISO TR 29922-2017">{{Cite web|url=https://www.iso.org/standard/67135.html|title=ISO/TR 29922:2017 - Natural gas -- Supporting information on the calculation of physical properties according to ISO 6976|website=www.iso.org|language=en|access-date=2018-04-20}}</ref>
* 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.<ref name="GPA 2145:2009">{{Cite web|url=http://infostore.saiglobal.com/store/Details.aspx?productID=1096564|title=GPA 2145:2009 {{!}} Table Of Physical Properties For Hydro... {{!}} SAI Global|last=saiglobal.com|website=infostore.saiglobal.com|access-date=2018-04-20}}</ref>

== References ==
<references />

Category:Gases

{{statisticalmechanics-stub}}

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Adapted from the Wikipedia article [Gas composition](https://en.wikipedia.org/wiki/Gas_composition) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Gas_composition?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.