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.

Components in dry air		Volume ratio = Molar ratio compared to dry air		Molar mass	Molar mass in air		Atmospheric boiling point
Name	Formula	[mol/molair]	[vol%]	[g/mol], [kg/kmol]	[g/molair], [kg/kmolair]	[wt%]	[K]	[°C]	[°F]
Nitrogen	N2	0.78084	78.084	28.013	21.873983	75.52	77.4	-195.8	-320.4
Oxygen	O2	0.20946	20.946	31.999	6.702469	23.14	90.2	-183.0	-297.3
Argon	Ar	0.00934	0.934	39.948	0.373114	1.29	87.3	-185.8	-302.5
Carbon dioxide	CO2	0.00033	0.033	44.010	0.014677	0.051	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

• 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