Air - Molecular Weight and Composition

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.

Components in Dry Air

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:

M_mixture = (x₁*M₁ + ......+ x_n*M_n) (1)

where

x_i= mole fractions of each gas
M_i = the molar mass of each 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.

For full table with Molar mass an Molar mass in Air - rotate the screen!

Components in dry air		Volume ratio = Molar ratio, compared to dry air		Molar Mass	Molar mass in air
Name	Formula	[mol/mol_air]	[vol %]	[g/mol] [kg/kmol]	[g/mol_air] [kg/kmol_air]	[wt %]
Nitrogen	N₂	0.78084	78.084	28.013	21.873983	75.52
Oxygen	O₂	0.20946	20.946	31.999	6.702469	23.14
Argon	Ar	0.00934	0.934	39.948	0.373114	1.29
Carbon dioxide	CO₂	0.00033	0.033	44.010	0.014677	0.051
Neon	Ne	0.00001818	0.001818	20.180	0.000367	0.0013
Helium	He	0.00000524	0.000524	4.003	0.000021	0.00007
Methane	CH₄	0.00000179	0.000179	16.042	0.000029	0.00010
Krypton	Kr	0.0000010	0.0001	83.798	0.000084	0.00029
Hydrogen	H₂	0.0000005	0.00005	2.016	0.000001	0.000003
Xenon	Xe	0.00000009	0.000009	131.293	0.000012	0.00004
Average molar mass of air					28.9647

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 (Pa)

R_air = 287.05 = 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 (= 273.15 K) can be calculated as

ρ = 101325 [Pa] / (287.05 [J/kg K]*273.15 [K])

= 1.292 [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.

moist air - temperature and saturation pressure

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

Related Documents

Air - Composition and Molecular Weight - Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.
Air - Density vs. Pressure and Temperatures - Air density at pressure ranging 1 to 10 000 bara (14.5 - 145000 psi) and constant selected temperatures.
Air - Density, Specific Weight and Thermal Expansion Coefficient vs. Temperature and Pressure - Online calculator, figures and tables showing density, specific weight and thermal expansion coefficients of air at temperatures ranging -100 to 1600 °C (-140 to 2900 °F) at atmospheric and higher pressure - Imperial and SI Units.
Air - Diffusion Coefficients of Gases in Excess of Air - Diffusion coefficients (D₁₂) for gases in large excess of air at temperatures ranging 0 - 400 °C.
Air - Dynamic and Kinematic Viscosity - Online calculator, figures and tables with dynamic (absolute) and kinematic viscosity for air at temperatures ranging -100 to 1600°C (-150 to 2900°F) and at pressures ranging 1 to 10 000 bara (14.5 - 145000 psia) - SI and Imperial Units.
Air - Prandtl Number - Prandtl number for air vs. temperature and pressure.
Air - Properties at Gas-Liquid Equilibrium Conditions - Properties of air change along the boiling and condensation curves (temperature and pressure between triple point and critical point conditions). An air phase diagram included.
Air - Specific Heat Ratio - Specific Heat Ratio of air at temperatures ranging -40 - 1000^oC (-40 - 1500^oF) at standard atmospheric pressure - Imperial and SI Units.
Air - Specific Heat vs. Pressure at Constant Temperature - Figures and tables with isobaric (Cp) and isochoric (Cv) specific heat of air at constant temperature and pressure ranging 0.01 to 10000 bara.
Air - Specific Heat vs. Temperature at Constant Pressure - Online calculator with figures and tables showing specific heat (Cp and Cv) of dry air vs. temperature and pressure. SI and imperial units.
Air - Speed of Sound vs. Temperature - Speed of sound in air at standard atmospheric pressure with temperatures ranging -40 to 1000^oC (-40 to 1500^oF) - Imperial and SI Units.
Air - Thermal Conductivity vs. Temperature and Pressure - Online calculator with figures and tables showing air thermal conductivity vs. temperature and pressure. SI and imperial units.
Air - Thermal Diffusivity vs. Temperature and Pressure - Figures and tables withdry air thermal diffusivity vs. temperarure and pressure. SI and Imperial units.
Air - Thermophysical Properties - Thermal properties of air at different temperatures - density, viscosity, critical temperature and pressure, triple point, enthalpi and entropi, thermal conductivity and diffusivity and more.
Dry Air - Thermodynamic and Physical Properties - Thermodynamic properties of dry air - specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, Prandtl number, density and kinematic viscosity at temperatures ranging 175 - 1900 K.
Dry Air and Water Vapor - Density and Specific Volume vs. Temperature - Imperial Units - Density and specific volume of dry air and water vapor at temperatures ranging 225 to 900 degF (107 to 482 degC).
Gas Mixtures - Properties - Gas mixtures and the ideal gas law, mass calculations, the individual gas constant and density.
Gases - Densities - Densities and molecular weights of common gases like acetylene, air, methane, nitrogen, oxygen and others.
Gases - Molar Specific Heats - Molar specific heats for some common gases at constant volume.
Humid Air vs. the Ideal Gas Law - Pressure, temperature and volume in a perfect ideal gas like moist air (air with water vapor).
Moist Air - Density vs. Water Content and Temperature - Density of the mix of dry air and water vapor - moist humid air.
Moist Air - Mole Fraction of Water Vapor - Mole fraction of water vapor is the ratio of water molecules to air and water molecules.
Mole and the Avagadro's Number - The mole is the SI base unit for an amount of a substance.
Molecular Weight of Substances - Definition and molecular weight (molar mass) of some common substances.
Nitrogen - Thermophysical Properties - Chemical, Physical and Thermal Properties of Nitrogen - N₂.
Non-ideal gas - Van der Waal's Equation and Constants - The van der Waals constants for more than 200 gases used to correct for non-ideal behavior of gases caused by intermolecular forces and the volume occupied by the gas particles.
Oxygen - Thermophysical properties - Chemical, Physical and Thermal Properties of Oxygen - O₂.
The Ideal Gas Law - The relationship between volume, pressure, temperature and quantity of a gas, including definition of gas density.
Total and Partial Pressure - Dalton's Law of Partial Pressures - How to calculate total pressure and partial pressures for gas mixtures from Ideal Gas Law.
Universal and Individual Gas Constants - The Universal and Individual Gas Constants in fluid mechanics and thermodynamics. Individual gas constants for the most common gases.

Air - Molecular Weight and Composition

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.

Components in Dry Air

Air Density

Water Vapor

Related Topics

Related Documents

Share

Search Engineering ToolBox

SketchUp Extension - Online 3D modeling!

About Engineering ToolBox!

Privacy

Advertise in the ToolBox

Topics

Unit Converters