Q:

# How heavy is air?

A:

One mole of air, containing no water vapor, weighs 28.96 grams. The addition of water vapor decreases the weight of air by up to a gram.

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Air is mainly comprised of nitrogen, oxygen and argon, with negligible amounts of other gases. The percent composition is 78 percent nitrogen, 21 percent oxygen and 1 percent argon. The molar mass is calculated by finding the sum of the atomic masses of each compound accounting for the varying amounts. Therefore, one mole of air contains 0.78 mole of nitrogen, 0.21 mole of oxygen and 0.1 mole of argon. Nitrogen and oxygen naturally form pairs of atoms in gases, meaning the atomic mass must be doubled. One mole of air contains 21.84 grams nitrogen (0.78 x 28), 6.72 grams oxygen (0.21 x 32) and 0.4 gram argon (0.01 x 40).

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