Molecular Mass Air

Overview

The molecular weight (molar mass) of dry air is the weighted average of its component gases by volume fraction. The average molar mass is 28.9647 g/mol (kg/kmol).

Average Molar Mass Formula

$M_{air} = \sum_{i} x_i \cdot M_i$

Where $x_i$ is the volume (mole) fraction of gas $i$ and $M_i$ is its molar mass (g/mol).

Air Density from Ideal Gas Law:

$\rho = \frac{P \cdot M_{air}}{R \cdot T}$

Symbol	Description	Value / Unit
$M_{air}$	Molar mass of dry air	28.9647 g/mol
$x_i$	Volume (mole) fraction of component $i$	—
$M_i$	Molar mass of component $i$	g/mol
$\rho$	Air density	kg/m³
$P$	Absolute pressure	Pa
$R$	Universal gas constant	8.314 J/(mol·K)
$T$	Absolute temperature	K

Air Composition Data

Important Properties and Effects

Water Vapor Effect: Water vapor (H₂O, molar mass ~18 g/mol) is lighter than the primary components of dry air (N₂, O₂). Its presence dilutes other gases, making humid air less dense than dry air at the same temperature and pressure.

Density Note: The calculated average molar mass for dry air is 28.9647 g/mol (or kg/kmol).

Interactive Charts

Major Components of Dry Air (Volume %)

Minor Components of Dry Air (Volume %)

Additional Engineering Notes

Components in Dry Air

Air Density

Water Vapor

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.

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 aprox. 32 g/mol and the molar mass of nitrogen gas is aprox. 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:

The molar mass of dry air with oxygen, nitrogen and the other components as indicated below is 28.9647 g/mol. Composition and content of each gas in air is given in the figures and the table below.

See also Air Density at varying pressure, Density and specific weight at varying temperature, Diffusion Coefficients for Gases in Air, Dynamic (absolute) and kinematic viscosity, Prandtl Number, Specific heat at varying temperature and Specific heat at varying pressure, Thermal Conductivity, Thermal Diffusivity, Properties at gas-liquid equilibrium conditions and Air properties, for other properties of air.

Example: The density of dry air at atmospheric pressure 101.325 kPa (101325 Pa) and 0 oC (273.15 K) can be calculated as

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 O2with 32 g/mol and N2with 28 g/mol .

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.

References

• Original Source