Molar Masses of Gasses

In a recent posts, we discussed how the amount of humidity in the air affects the air density. To do this, we explained that because the molar mass of water vapor (H2O) is less than that of the composition of dry air, humid air would therefore be less dense than dry air given that the temperature and pressure remain constant. In this post we would like to explain how we came to the conclusion that water vapor is “lighter” than dry air.

Dry air is made up of roughly 78% nitrogen, 21% oxygen and 1% different gases such as argon, carbon dioxide, methane, nitrogen dioxide, helium, and many others, however, Argon makes up the .9% of the last 1%. Lets take a look at the periodic table in order to compare the molar masses of these different gases.

Each box represents a different element. The number below the letter represents the molar mass of the element. The molar mass tells how many grams of the substance are in one mole. A mole represents 6.022×10^23 atoms of the element. This number may seem arbitrary, but it represents the number of atoms in 12 grams of carbon-12. For instance, carbon (C) contains 12 grams per mole and nitrogen (N) contains 14 grams per mole.

Let’s now take a look at the mass per unit volume of dry air and saturated air (water vapor). We can reasonably assume that the mass of dry air is equal to the proportional masses of nitrogen, oxygen, and argon because they make up 99.9% of dry air. Nitrogen and oxygen come in diatomic form, meaning that a molecule of oxygen contains two oxygen atoms, and a molecule of nitrogen contains two nitrogen atoms. Argon comes in a monotomic form meaning that one atom of argon is also one molecule. So, lets do some math and see how the mass of dry air stacks up against the mass of saturated air:

These tables conclude that as the ratio of water vapor increases in a unit volume of atmospheric air, the density will decrease because water vapor is “heavier” than dry air.

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