We know average volume proportions between compounds in dry air. Here are the more important (in terms of proportions) ones:

Pie chart

How can we calculate mass proportions between these gases?

I'm considering a more general case, not only the exact values from the chart.


1 Answer 1


Air can be considered to be an ideal gas. This means that the volume percents of the given gases are also the mole percents of those same gasses. Accordingly, you need to find the weight fraction of each of the given gases. This is done by multiplying the mole fraction by the molar mass for each gas to arrive at a pseudo-mass. Do this for all gases and add up all of the pseudo-masses to get a total pseudo-mass. Divide all of the pseudo-masses by the total pseudo-mass to get mass fractions.

For example, for a case where there is 0.21 mole fraction of oxygen and 0.79 mole fraction of nitrogen, the oxygen pseudo-mass is 0.21*32 (oxygen is diatomic with a molar mass of 32), which equals 6.72. The nitrogen pseudo-mass is 0.79 * 28 (nitrogen is diatomic with a molar mass of 28), which equals 22.12. The total pseudo-mass is 28.84, which is the average molar mass of air with the stated composition. Divide the individual pseudo-masses by 28.84 to arrive at 0.233 weight fraction oxygen and 0.767 weight fraction nitrogen.

  • $\begingroup$ Thank you! May you please explain what would be a reverse transition too? It seems to me that to calculate volume of one gas having the mass proportions, we need average molar mass of the air, which is derived from gases' volumes. Thanks in advance! $\endgroup$
    – Elgirhath
    Mar 18, 2019 at 19:45
  • $\begingroup$ Reverse transition - assume 100 g total mass. Multiply this total by each mass fraction to get mass of each component. Divide each mass fraction by the appropriate molar mass to get moles of each component. Divide each mole by total moles to get molar fraction, which is volume fraction for an ideal gas. $\endgroup$ Mar 18, 2019 at 19:49
  • $\begingroup$ Thank you! Upvotes will be added when I reach 15 reputation ;) $\endgroup$
    – Elgirhath
    Mar 18, 2019 at 20:00

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