Air - Molecular Mass

Dry air is a mixture of gases where the molecular weight can be calculated by adding the weight of each component

The molecular mass of a substance is the mass of one molecule of the substance. Molecular mass is expressed with the unit u (equal to 1/12 the mass of one atom of carbon-12) and the molecular mass can be calculated by summarizing the atomic masses of all the atoms in the molecule.

Components in Dry Air

The two most dominant components in dry air are Oxygen and Nitrogen. Oxygen has an atomic unit mass of 16 and Nitrogen has an atomic units mass of 14. Since both of these elements are diatomic in air - O2 and N2, the molecular mass of Oxygen is 32 and the molecular mass of Nitrogen is 28.

Air is a mixture of gases and the total mass can be estimated by adding the weight of all major components as shown below:

Components in Dry AirVolume Ratio compared to Dry AirMolecular Mass - M
Molecular Mass in Air
Oxygen 0.2095 32.00 6.704
Nitrogen 0.7809 28.02 21.88
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
Total Molecular Mass of Air 28.97
  • 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)


p = pressure (kPa)

R = 286.9 = individual gas constant (J/kg K)

T = absolute temperature (K)

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 at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 18 atomic units. With 18 atomic units water vapor is lighter than diatomic Oxygen with 32 units and diatomic Nitrogen with 28 units.

  • Note! Water vapor in air will replace 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

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