The Gas composition of any

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

can be characterised by listing the

pure substance A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be com ...

s it contains, and stating for each substance its proportion of the gas mixture's

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

count.Nitrogen 78.084 Oxygen 20.9476 Argon Ar 0.934 Carbon Dioxide 0.0314

Gas composition of air

To give a familiar example,

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

has a composition of: 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 In chemistry, the molar mass () (sometimes called molecular weight or formula weight, but see related quantities for usage) of a chemical substance ( element or compound) is defined as the ratio between the mass () and the amount of substance ...

. 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 In physics and chemistry, an equation of state is a thermodynamic equation relating state variables, which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature, or internal energy. Most mod ...

. 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 kg·kmol-1. * 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.

References

Gases {{statisticalmechanics-stub