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

As we know that mass = number of moles × molar mass Molar mass of N = 14 Molar mass of N2 =14 × 2 = 28 Mass of N2 = 0.5 × 28 = 14 g. Nitrogen is a colorless, odorless, and tasteless gas which is composed of diatomic molecules N2. It constitutes 78% of the air by volume. At 0°C and 1 atmosphere pressure, a liter has a mass of 1.2506 grams. The gas condenses to a colorless liquid at 77.25 K and to a white solid at 63.3 K. Nitrogen is chemically unreactive.

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.

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.

War thunder a4e. 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:

Mmixture = (x1*M1 + ...+ xn*Mn) (1)


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

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

See also AirDensity 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.

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

Components in dry airVolume ratio = Molar ratio,
compared to dry air
Molar Mass
Molar mass in air
NameFormula[mol/molair][vol %][g/mol]
[wt %]
Carbon dioxideCO20.000330.03344.0100.0146770.051
Average molar mass of air28.9647
  • 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)


P = pressure (Pa)

Rair = 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])

Atomic weight of nitrogen

= 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.

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
  • Material Properties - Material properties for gases, fluids and solids - densities, specific heats, viscosities and more
  • Fluid Mechanics - The study of fluids - liquids and gases. Involves velocity, pressure, density and temperature as functions of space and time
  • Gases and Compressed Air - Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves

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 at varying pressure and constant temperatures - Figures and tables showing changes in air density at pressure varying from 1 to 10 000 bara (14.5 - 145000 psi) and constant, selected temperatures. Figures are given in different scales.
  • Air - Density, Specific Weight and Thermal Expansion Coefficient at Varying Temperature and Constant Pressures - Online calculator, figures and tables showing density, specific weight and thermal expansion coefficient 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 temperature varying from 0 - 400 °C
  • Air - Dynamic and Kinematic Viscosity - Online calculator, figures and tables showing dynamic (absolute) and kinematic viscosity of air at temperatures ranging from -100 to 1600°C (-150 to 2900°F) at pressure ranging from 1 to 10 000 bara (14.5 - 145000 psia) - SI and Imperial Units
  • Air - Prandtl Number - Figures and table showing changes in Prandtl number for air with changes in temperature and pressure
  • Air - Properties at Gas-Liquid Equilibrium Conditions - Figures and tables showing how the properties of air changes along the boiling and condensation curves (temperature and pressure between triple point and critical point conditions). An air phase diagram are also given.
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  • Air - Specific Heat at Constant Temperature and Varying Pressure - Figures and table showing isobaric (Cp) and isochoric (Cv) specific heat of air at constant temperature and varying pressure ranging 0.01 to 10000 bara
  • Air - Thermal Conductivity - Online calculator, figures and tables showing air thermal conductivity at varying temperature and pressure, SI and imperial units.
  • Air - Thermal Diffusivity - Figures and tables showing dry air thermal diffusivity at varying temperarure and pressure, SI and Imperial units
  • Air - Thermophysical Properties - Thermal properties of air - density, viscosity, critical temperature and pressure, triple point, enthalpi and entropi, thermal conductivity and diffusicity, and more
  • Air Specific Heat Ratio - Specific Heat Ratio of air at temperatures from -40 - 1000oC (-40 - 1500oF) at standard atmospheric pressure - Imperial and SI Units
  • Density of Moist Humid Air - Density of dry air and water vapor - moist or humid air
  • Dry Air and Water Vapor - Density and Specific Volume - Imperial Units - Density and specific volume of dry air and water vapor at temperatures ranging 225 to 900 degF (107 to 482 degC)
  • Dry Air Properties - Dry air properties at temperatures ranging 175 - 1900 K - specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, Prandtl number, density and kinematic viscosity
  • Gas Mixture Properties - Special care must be taken for gas mixtures when using the ideal gas law, calculating the mass, the individual gas constant or the density
  • Gases - Densities - Densities and molecular weights of some common gases - acetylene, air, methane, nitrogen, oxygen and others .
  • Gases - Molar Specific Heat - Molar specific heats of gases at constant volume
  • Humid Air and the Ideal Gas Law - Pressure, temperature and volume for an ideal or perfect gas like air with water vapor - or moist air
  • Ideal Gas Law - The relations between volume, pressure, temperature and quantity of a gas, including definition of density of a gas
  • Mole and Avagadro's Number - The mole is the SI base unit for amount of substance
  • Mole Fraction of Water Vapor in Moist Air - Mole fraction of water vapor is the ratio of water molecules - to air and water molecules
  • Molecular Weight of some Common 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 - Listing of 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
  • Speed of Sound in Air - Speed of sound in air at temperatures from -40 to 1000oC (-40 to 1500oF) at standard atmospheric pressure - Imperial and SI Units
  • 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 constant is given for the most common gases.

Atomic Mass Of N2o

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