The molecular mass of a substance is the mass of one molecule of that substance and is expressed in the terms of the molecular mass unit u (equal to 1/12 the mass of one atom of carbon-12). The molecular mass can be calculated as the sum of the atomic masses of all the atoms of any molecule.
The two most dominant components in dry air are Oxygen and Nitrogen. Oxygen has a 16 atomic unit mass and Nitrogen has a 14 atomic units mass. 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 Air||Volume Ratio compared to Dry Air||Molecular Mass - M
|Molecular Mass in Air|
|Total Molecular Mass of Air||28.97|
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 oK)
T = absolute temperature (oK)
Density at atmospheric pressure 101.325 kPa (101325 Pa) and 0oC can be calculated as
ρ = (101325 Pa) / ((286.9 J/kg oK) (273 oK))
= 1.294 kg/m3
Water vapor is almost always presence in the 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.