By Pauline Gill
Updated Aug 30, 2022

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The molar mass of any substance—solid, liquid, or gas—is the mass of one mole, i.e., 6.022 × 10²³ entities, of that substance. It is determined by the sum of the atomic masses of its constituent atoms. For a mixture like air, the molar mass is the weighted average of each component’s molecular weight, weighted by its volumetric fraction in the mixture.

Step 1 – Identify the Major Constituents

Air is composed primarily of the following gases (volumetric fractions relative to 1 unit of air):

• Nitrogen 78.09 % (0.7809)
• Oxygen 20.95 % (0.2095)
• Argon 0.933 % (0.00933)
• Carbon dioxide 0.03 % (0.0003)
• Other trace gases: neon 0.000018, helium 0.000005, krypton 0.000001, hydrogen 0.0000005, xenon 0.09 × 10⁻⁶

Step 2 – Multiply by Molecular Weights

Because nitrogen (N₂) and oxygen (O₂) are diatomic, their molecular weights are twice their atomic masses: 28.014 g mol⁻¹ for N₂ and 32.00 g mol⁻¹ for O₂. The calculations are as follows:

• Nitrogen: 28.014 × 0.7809 = 21.876
• Oxygen: 32.00 × 0.2095 = 6.704
• Argon: 39.94 × 0.00933 = 0.3726
• Carbon dioxide: 44.01 × 0.0003 = 0.013
• Neon: 20.18 × 0.000018 = 3.6324 × 10⁻⁴
• Helium: 4.00 × 0.000005 = 2.0 × 10⁻⁵
• Krypton: 83.8 × 0.000001 = 8.38 × 10⁻⁵
• Hydrogen: 2.02 × 0.0000005 = 1.01 × 10⁻⁶
• Xenon: 131.29 × 0.09 × 10⁻⁶ = 1.18 × 10⁻⁵

Step 3 – Sum the Contributions

Adding all the weighted molecular weights gives a molar mass of air of 28.9656 g mol⁻¹. This means that one mole of dry air (6.022 × 10²³ molecules) weighs 28.9656 grams under standard temperature and pressure (STP) of 60 °F and 14.696 psi absolute. At STP, that mole occupies 22.4 L, or 0.7910 ft³.

TL;DR

Maintain proper ventilation to keep indoor CO₂ low and O₂ levels healthy. Understanding air’s molar mass helps engineers design HVAC systems and assess atmospheric composition.

Warning

Cryogenic gases and liquids are extremely cold and can freeze skin in seconds upon contact. Handle them with appropriate protective equipment.