Air - Composition and Molecular Weight

Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.

Components in Dry Air

Air is a mixture of several gases, where the two most dominant components in dry air are 21 vol% oxygen and 78 vol% nitrogen. Oxygen has a molar mass of 15.9994 g/mol and nitrogen has a molar mass of 14.0067 g/mol. Since both of these elements are diatomic in air - O₂ and N₂, the molar mass of oxygen gas is 32 g/mol and the molar mass of nitrogen gas is 28 g/mol.

The average molar mass is equal to the sum of the mole fractions of each gas multiplied by the molar mass of that particular gas:

M_mixture = (x₁*M₁ + ......+ x_n*M_n) (1)

where

x_i= mole fractions of each gas
M_i = the molar mass of each gas

The molar mass of dry air is 28.9647 g/mol. Composition and content of each gas in air is given in the figures and the table below.

Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.

For full table - rotate the screen!

Components in dry air		Volume ratio = Molar ratio compared to dry air		Molar mass	Molar mass in air		Atmospheric boiling point
Name	Formula	[mol/mol_air]	[vol%]	[g/mol], [kg/kmol]	[g/mol_air], [kg/kmol_air]	[wt%]	[K]	[°C]	[°F]
Nitrogen	N₂	0.78084	78.084	28.013	21.872266	75.511	77.4	-195.8	-320.4
Oxygen	O₂	0.20946	20.946	31.999	6.701942	23.14	90.2	-183.0	-297.3
Argon	Ar	0.00934	0.934	39.948	0.373025	1.29	87.3	-185.8	-302.5
Carbon dioxide¹⁾	CO₂	0.000412	0.0412	44.010	0.018132	0.063	194.7	-78.5	-109.2
Neon	Ne	0.00001818	0.001818	20.180	0.000367	0.0013	27.2	-246.0	-410.7
Helium	He	0.00000524	0.000524	4.003	0.000021	0.00007	4.2	-269.0	-452.1
Methane	CH₄	0.00000179	0.000179	16.042	0.000029	0.00010	111.7	-161.5	-258.7
Krypton	Kr	0.0000010	0.0001	83.798	0.000084	0.00029	119.8	-153.4	-244.0
Hydrogen	H₂	0.0000005	0.00005	2.016	0.000001	0.000003	20.3	-252.9	-423.1
Xenon	Xe	0.00000009	0.000009	131.293	0.000012	0.00004	165.1	-108.1	-162.5
Average molar mass of air					28.9647

¹⁾ According NASA CO₂ level in 1960 aprox. 320 ppm, 1970 aprox. 328 ppm, 1980 aprox. 341 ppm, 1990 aprox. 356 ppm, 2000 aprox. 372 ppm, 2010 aprox. 390 ppm and 2020 aprox. 412 ppm

The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature
The composition of air is unchanged until elevation of approximately 10.000 m
The average air temperature diminishes at the rate of 0.6^oC for each 100 m vertical height
"One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0^oC sea level and at standard gravity (32.174 ft/sec²)

Other components in air

Sulfur dioxide - SO₂ - 1.0 parts/million (ppm)
Nitrous oxide - N₂O - 0.5 parts/million (ppm)
Ozone - O₃ - 0 to 0.07 parts/million (ppm)
Nitrogen dioxide - NO₂ - 0.02 parts/million (ppm)
Iodine - I₂ - 0.01 parts/million (ppm)
Carbon monoxide - CO - 0 to trace (ppm)
Ammonia - NH₃ - 0 to trace (ppm)

Common Pressure Units frequently used as alternative to "one Atmosphere"

76 Centimeters (760 mm) of Mercury
29.921 Inches of Mercury
10.332 Meters of Water
406.78 Inches of Water
33.899 Feet of Water
14.696 Pound-Force per Square Inch
2116.2 Pounds-Force per Square Foot
1.033 Kilograms-Force per Square Centimeter
101.33 kiloPascal

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Air - Composition and Molecular Weight

Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.

Components in Dry Air

Common Pressure Units frequently used as alternative to "one Atmosphere"

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