How do I calculate the fractional concentration of gases in a mixture?

The concentration of a single gas, A, in the mixture, M, can be represented in terms of its fractional concentration: the number of molecules of gas A divided by the number of all the molecules in the gas mixture (the amount of gas A plus the amount of gas B plus the amount of gas C).

The atmosphere is a mixture of gases, including nitrogen, oxygen, carbon dioxide, argon, and water vapor. The amounts of argon and carbon dioxide are minimal. Under normal ambient (dry) conditions, there is also little water vapor.

Just as we did for the generic gas mixture above, the concentrations of gases in the atmosphere can be expressed as fractional concentrations. But for these purposes, we can ignore the concentrations of all gases but oxygen and nitrogen. And, since this is the air we breathe in, we can express them as fractional concentrations of inspired gas, X (F_IX).

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