Air - Molecular Weight

Dry air is a mixture of gases where the average molecular weight (or molar mass) can be calculated by adding the weight of each component

The molecular weight (or molar mass) of a substance is the mass of one mole of the substance, and can be calculated by summarizing the molar masses of all the atoms in the molecule.

Components in Dry Air

Air is a mixture of several gases, where the two most dominant components in dry air are oxygen and nitrogen. Oxygen has a molar mass of 16 g/mol and nitrogen has a molar mass of 14 g/mol. Since both of these elements are diatomic in air - O2 and N2, the molar mass of oxygen is 32 g/mol and the molar mass of nitrogen is 28 g/mol.

Air is a mixture of gases, and 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

Components in Dry AirVolume Ratio compared to Dry AirMolar Mass - M
[g/mol]
Molar Mass in Air
[g/mol]
xygen 0.2095 32.00 6.704
Nitrogen 0.7809 28.02 21.881
Carbon dioxide 0.0003 44.01 0.013
Hydrogen 0.0000005 2.02 0
Argon 0.00933 39.94 0.373
Neon 0.000018 20.18 0
Helium 0.000005 4.00 0
Krypton 0.000001 83.8 0
Xenon 0.09 10-6 131.29 0
Average molar mass of air 28.971
  • 1 kg = 2.2046 lb

Air Density

The density of dry air can be calculated with the Ideal Gas Law 

ρ = p / (R T)                              (1)

where

p = pressure (kPa)

R = 286.9 = individual gas constant [J/kg K]

T = absolute temperature [K]


Example: The density of dry air at atmospheric pressure 101.325 kPa (101325 Pa) and 0 oC can be calculated as

ρ = 101325 [Pa] / (286.9 [J/kg K]*273 [K])

    = 1.294 [kg/m3]

Water Vapor

Water vapor is almost always present in air. The content may vary and the maximum amount possible of water vapor in dry air depends on the temperature of the air.

Water vapor - H2O - is composed of one oxygen atom and two hydrogen atoms. Hydrogen is the lightest element with a molar mass of 1 g/mol,  while oxygen has 16 g/mol. Thus, the water vapor atom has an molar mass of 18 g/mol, and water vapor is lighter than O2 with 32 g/mol and N2 with 28 g/mol.

  • Note! Water vapor in air will dilute the other gases and reduce the total density of the mixture. Dry air is more dense than humid air!

The vapor in air may saturate to droplets when temperature is decreased or pressure is increased. 

Humid air containing water molecules as liquid - droplets - may be more dense than dry air or humid air containing water only as vapor.  

Related Topics

  • Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content
  • Material Properties - Material properties for gases, fluids and solids - densities, specific heats, viscosities and more

Related Documents

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