Air - Molecular Weight and Composition | MEPKit Engineering Tools

Component	Formula	Volume Ratio (mol/molair)	Molar Mass (g/mol)	Molar Mass in Air (g/molair)	Weight % (wt%)
Nitrogen	N2	0.78084	28.013	21.873983	75.52
Oxygen	O2	0.20946	31.999	6.702469	23.14
Argon	Ar	0.00934	39.948	0.373114	1.29
Carbon dioxide	CO2	0.00033	44.010	0.014677	0.051
Neon	Ne	0.00001818	20.180	0.000367	0.0013
Helium	He	0.00000524	4.003	0.000021	0.00007
Methane	CH4	0.00000179	16.042	0.000029	0.00010
Krypton	Kr	0.0000010	83.798	0.000084	0.00029
Hydrogen	H2	0.0000005	2.016	0.000001	0.000003
Xenon	Xe	0.00000009	131.293	0.000012	0.00004
Average molar mass of air				28.9647	100.00

Calculation Method

The average molar mass of air is calculated using the formula:

Mmixture = x₁M₁ + ... + xₙMₙ

Where:

xᵢ = mole fractions of each gas
Mᵢ = the molar mass of each gas

Key Components

Dry air is composed primarily of:

78.084 vol% Nitrogen (N₂) with molar mass ~28 g/mol
20.946 vol% Oxygen (O₂) with molar mass ~32 g/mol
0.934 vol% Argon (Ar) with molar mass ~40 g/mol
Trace amounts of CO₂, Ne, He, CH₄, Kr, H₂, and Xe

Average Molar Mass of Dry Air: 28.9647 g/mol

Air Density

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

ρ = P / (R·T)

Where:

P = pressure (Pa)
R_air = 287.05 J/kg·K (individual gas constant)
T = absolute temperature (K)

Example: At atmospheric pressure (101.325 kPa) and 0°C (273.15 K): ρ = 1.292 kg/m³

Water Vapor Note

⚠️ Important: Water vapor (H₂O) is lighter than both O₂ and N₂, so dry air is denser than humid air. Water vapor dilutes the other gases and reduces the total density of the mixture.