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# Air - Composition and Molecular Weight

## Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.

### 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 32 g/mol and the molar mass of nitrogen gas is 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 is 28.9647 g/mol. Composition and content of each gas in air is given in the figures and the table below.

Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.

For full table - rotate the screen!

 Components in dry air Volume ratio = Molar ratio compared to dry air Molar mass Molar mass in air Atmospheric boiling point Name Formula [mol/mol air ] [vol%] [g/mol], [kg/kmol] [g/mol air ], [kg/kmol air ] [wt%] [K] [°C] [°F] Nitrogen N 2 0.78084 78.084 28.013 21.872266 75.511 77.4 -195.8 -320.4 Oxygen O 2 0.20946 20.946 31.999 6.701942 23.14 90.2 -183.0 -297.3 Argon Ar 0.00934 0.934 39.948 0.373025 1.29 87.3 -185.8 -302.5 Carbon dioxide 1) CO 2 0.000412 0.0412 44.010 0.018132 0.063 194.7 -78.5 -109.2 Neon Ne 0.00001818 0.001818 20.180 0.000367 0.0013 27.2 -246.0 -410.7 Helium He 0.00000524 0.000524 4.003 0.000021 0.00007 4.2 -269.0 -452.1 Methane CH 4 0.00000179 0.000179 16.042 0.000029 0.00010 111.7 -161.5 -258.7 Krypton Kr 0.0000010 0.0001 83.798 0.000084 0.00029 119.8 -153.4 -244.0 Hydrogen H 2 0.0000005 0.00005 2.016 0.000001 0.000003 20.3 -252.9 -423.1 Xenon Xe 0.00000009 0.000009 131.293 0.000012 0.00004 165.1 -108.1 -162.5 Average molar mass of air 28.9647

1) According NASA CO2level in 1960 aprox. 320 ppm, 1970 aprox. 328 ppm, 1980 aprox. 341 ppm, 1990 aprox. 356 ppm, 2000 aprox. 372 ppm, 2010 aprox. 390 ppm and 2020 aprox. 412 ppm

• The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature
• The composition of air is unchanged until elevation of approximately 10.000 m
• The average air temperature diminishes at the rate of 0.6 oC for each 100 m vertical height
• "One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0 oC sea level and at standard gravity ( 32.174 ft/sec2 )

Other components in air

• Sulfur dioxide - SO2- 1.0 parts/million (ppm)
• Nitrous oxide - N2O - 0.5 parts/million (ppm)
• Ozone - O3 - 0 to 0.07 parts/million (ppm)
• Nitrogen dioxide - NO2- 0.02 parts/million (ppm)
• Iodine - I2- 0.01 parts/million (ppm)
• Carbon monoxide - CO - 0 to trace (ppm)
• Ammonia - NH3 - 0 to trace (ppm)

### Common Pressure Units frequently used as alternative to " one Atmosphere"

• 76 Centimeters (760 mm) of Mercury
• 29.921 Inches of Mercury
• 10.332 Meters of Water
• 406.78 Inches of Water
• 33.899 Feet of Water
• 14.696 Pound-Force per Square Inch
• 2116.2 Pounds-Force per Square Foot
• 1.033 Kilograms-Force per Square Centimeter
• 101.33 kiloPascal

## Related Topics

• ### Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.
• ### Fluid Mechanics

The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
• ### Gases and Compressed Air

Properties of air, LNG, LPG and other common gases. Pipeline capacities and sizing of relief valves.
• ### Material Properties

Properties of gases, fluids and solids. Densities, specific heats, viscosities and more.

## Related Documents

• ### Air - Density vs. Pressure and Temperature

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 - Humidity Ratio

The mass of water vapor present in moist air - to the mass of dry air.
• ### Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.
• ### 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.
• ### 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 degC (-40 - 1500 degF) 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 °C (-40 to 1500 °F) - 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.
• ### Air Conditioning - Cooling of Air and Condensate Generated

Water may condensate when air is cooled in air conditioning systems.
• ### Carbon dioxide - Prandtl Number vs. Temperature and Pressure

Figures and table with changes in Prandtl number for carbon dioxide with changes in temperature and pressure.
• ### Carbon Dioxide - Thermophysical Properties

Chemical, physical and thermal properties of carbon dioxide. Phase diagram included.
• ### Compressed Air - Pressure Drop Diagrams, Metric Units

Pressure loss in compressed air pipe lines.
• ### 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.
• ### Humid Air and 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.
• ### Moist Air - Partial Pressure and Daltons Law

The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.
• ### Moist Air - Specific Volume

Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air
• ### Moist Air - Weight of Water Vapor

Weight of water vapor in air
• ### Nitrogen - Enthalpy, Internal Energy and Entropy vs. Temperature

Enthalpy, internal energy and entropy of Nitrogen as an ideal gas.
• ### Nitrogen - Density and Specific Weight vs. Temperature and Pressure

Online calculator, figures and tables showing density and specific weight of nitrogen, N2, at temperatures ranging from -175 to 1325 °C (-280 to 2400 °F) at atmospheric and higher pressure - Imperial and SI Units.
• ### Nitrogen - Prandtl number vs. Temperature and Pressure

Figures and tables showing Prandtl number of nitrogen at varying temperarure and pressure, SI and Imperial units.
• ### Nitrogen - Thermal Diffusivity vs. Temperature and Pressure

Figures and tables showing thermal diffusivity of nitrogen at varying temperarure and pressure, SI and Imperial units.
• ### Nitrogen - Thermophysical Properties

Chemical, Physical and Thermal Properties of Nitrogen - N2.
• ### Nitrogen Gas - Specific Heat vs. Temperature

Specific heat of Nitrogen Gas - N2 - at temperatures ranging 175 - 6000 K
• ### Oxygen - Density and Specific Weight vs. Temperature and Pressure

Online calculator, figures and tables showing density and specific weight of oxygen, O2, at varying temperature and pressure - Imperial and SI Units.
• ### Oxygen - Enthalpy, Internal Energy and Entropy vs. Temperature

Enthalpy, internal energy and entropy of oxygen as an ideal gas.
• ### Oxygen - Thermophysical properties

Chemical, Physical and Thermal Properties of Oxygen - O2.
• ### Oxygen Gas - Specific Heat vs. Temperature

Specific heat of Oxygen Gas - O2 - at temperatures ranging 175 - 6000 K.
• ### Solubility of Air in Water

The amount of air that can be dissolved in water decreases with temperature and increases with pressure.
• ### 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.
• ### U.S. Standard Atmosphere vs. Altitude

Properties of the US standard atmosphere ranging -5000 to 250000 ft altitude.

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## Citation

• The Engineering ToolBox (2003). Air - Composition and Molecular Weight. [online] Available at: https://www.engineeringtoolbox.com/air-composition-d_212.html [Accessed Day Month Year].

Modify the access date according your visit.

7.8.15

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