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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

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:

Mmixture = (x1*M1 + ......+ xn*Mn)                                        (1)

where

xi = mole fractions of each gas
Mi = 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/molair] [vol%] [g/mol],[kg/kmol] [g/molair],[kg/kmolair] [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 0.6oC 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 0oC 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

## Related Topics

• Air Psychrometrics - Moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content.
• Material Properties - Material properties of gases, fluids and solids - densities, specific heats, viscosities and more.
• Fluid Mechanics - The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
• Gases and Compressed Air - Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves.

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