What is the molecular mass or weight of air?

The below article talks about the molecular mass of air, the components that contribute towards this, along with some frequently asked questions about the molecular mass of air and other factors that affect the molecular mass of air. 

What is the molecular mass or weight of air?

The air around us is a mixture of several components and gases, and each of these brings in its own characteristics to the overall health of the air around us. The most dominants elements in dry air include oxygen, carbon dioxide, nitrogen, and argon; and combining these elements along with some trace particulates of other chemicals, the molecular mass of air can be calucuated to be approximately 28.9647 g / mol (TET, n.d.). 

Molecular mass

Molecular mass, a.k.a. Molar mass of a chemical element is the sum of the total mass in grams of all the atoms that make up a mole of a particular molecule. The unit used to measure is grams per mole. A mole is unit of measurement that aims to compre particles of any given chemical compound to its mass. 

Molar Mass (M)=GramsMoles kg/mol (or) g/mol 

SI unit for molar mass is kg/mol and other units that could be used to symbolise molar mass also includes, g/mol. 

The molar mass of any matter – solid, liquid, or gas, is the number of grams of the substance in its molecular (molar) form that contains 6.0221367 e23 atoms of the substance (Avogadro’s Number). This is explained as the mass of a substance being dependent on the molecular weight of the substance, which is determined by the number of protons and neutrons in the nucleus of any atom. Therefore, the molar mass of a substance such as air is dependent on the sum of all the volume fractions of each molecular component in air, times their individual molecular weights.

Calculating the molar mass of air

Air is a mixture of several gases, determining the molecular weight of air requires that we identify the most contributing elements to the air around us. The most dominant components in dry air around us include:

  • 21% of oxygen 
  • 78% of nitrogen
  • 0.934% of argon 
  • 0.03%of carbon dioxide

While the above 4 elements are the most dominant, the other minor elements in dry air include: 

  • Neon with 0.0018%
  • Helium with 0.0005%
  • Krypton with 0.0001% 
  • Hydrogen with 0.00005%
  • Methane with 0.0002%
  • Xenon with 0.000009% 

However, these elements are too small in volume to make significant molar mass changes in air. Therefore, they are usually ignored when calculating the molar mass of dry air (Gill P., 2018). 

Oxygen and Nitrogen are diatomic in air – O2 and N2, the molar mass of these gases can be explained as follows: 

The atomic / molecular weights are:

  • Nitrogen: 14.0067 x 2 = 28.0134 g/mol 
  • Oxygen: 15.9994 x 2 = 31.9988 g/mol 
  • Argon: 39.948 g/mol
  • Carbon dioxide: 44.01 g /mol

Weight of each gas by recognizing the percentage of volume they contribute towards the air mixture around us

  • Nitrogen: (78.084 / 100) x 28.0134 = 21.8739 g/mol
  • Oxygen: (20.946 / 100) x 31.9988 = 6.7025 g/mol
  • Argon: (0.934 / 100) x 39.948 = 0.373 g/mol
  • Carbon dioxide: (0.03 / 100) x 44.01 = 0.013203 g/mol

Add all the above values we get,

21.8739 + 6.7025 + 0.373 + 0.013203 = 28.96 g / mol

This is the molecular weight of the air (Byjus, n.d.)

Components in Dry Air

The below table shows extensive information of all contributing elements in air, including their volume in air, their molar mass in air and otherwise, and their boiling point under normal circumstances (TET, n.d.)

Components in dry airVolume Ratio (Molar Ratio in comparison to dry air)Molar MassMolar Mass in airBoiling Point
NameFormula[mol/molair][volume in %][g/mol][g/molair][weight in %][K][°C][°F]
Carbon dioxideCO20.0004120.041244.0100.0181320.063194.7-78.5-109.2
Average molar mass of air28.9647

Frequently Asked Questions (FAQs): What is the molecular weight of air?

What is Avagadro’s Number?

The number of units in one mole of any substance is called as the Avogadro’s number or the Avogadro’s constant. It is said to be 6.022140857×1023. The units of the substance in consideration could be electrons, ions, atoms, or molecules, of a solid, liquid, or gaseous matter, depending on the character of the reaction and the nature of the substance.

An illustration of the above definition would be as follows. For example, if you wanted to know the number of particles in 5 moles of a substance, the value would be:

= 5 x 6.023 x 1023 =  30.115 x 1023 particles (or) 3.0115 x 1024 particles

Is humid air “heavier” in molecular weight than dry air?

The molecular weight of dry air is 28.9647 g / mol.

The molecular weight of moist air is 18.015 g / mol.

The connotation of moist air may denote that it is refers to air conditions with higher water content, however, water content is always present in air in variable amounts. In comparing elements mole per mole, dry air is heavier than water vapor, and any mixture of dry air with water vapor will weigh less than just dry air. Therefore, the more water vapor in the air, the lower would the molecular weight be (Ackerman S. A. & Martin J., 2010) (Soares C., 2015). References 

Ackerman S. A. & Martin J. (2010, August 16). Is humid air “heavier” than dry air? The Why Files. Viewed on 01-22-2022. https://whyfiles.org/2010/the-weather-guys-heavy-air/index.html 

Byjus. (n.d.). What is Avagadro’s Number? Viewed on 01-22-2022. https://byjus.com/physics/avogadros-number/ 

Byjus. (n.d.). What is the molecular weight of air? Viewed on 01-22-2022. https://byjus.com/questions/what-is-the-molecular-weight-of-air/ 

Gill P. (2018, March 13). How to calculate molar mass of air. SCIENCING. Viewed on 01-22-2022. https://sciencing.com/calculate-air-volume-5146908.html 

Soares C. (2015). Performance, Performance Testing, and Performance Optimization. Chapter 10. Second Edition. Gas Turbines. pp. 533 – 636. Viewed on 01-22-2022. https://www.sciencedirect.com/topics/engineering/moist-air#:~:text=MW%20is%20the%20molecular%20weight%20of%20moist%20air.&text=RA%20is%20the%20gas%20constant,%2C%200.28705%20kJ%2Fkg%20K.&text=WAR%20is%20the%20ratio%20of%20water%20to%20dry%20air%2C%20by%20mass.&text=18.015%2C%2028.96%20are%20the%20molecular,water%20and%20dry%20air%20respectively

The Engineering ToolBox (TET). (n.d.). Air – Composition and Molecular Weight. Viewed on 01-22-2022. https://www.engineeringtoolbox.com/air-composition-d_212.html 

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