We are asked what is the mole fraction of oxygen gas in air.

O_{2} is 21% and N_{2} 79% by mass in air.

Assume 100 g air.

There will be 21 g O_{2} and 79g N_{2}.

Calculate moles each:

$\mathbf{mol}\mathbf{}{\mathbf{O}}_{\mathbf{2}}\mathbf{}\mathbf{=}\mathbf{}\mathbf{21}\overline{)\mathbf{}\mathbf{g}\mathbf{}{\mathbf{O}}_{\mathbf{2}}\mathbf{}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{}{\mathbf{O}}_{\mathbf{2}}}{\mathbf{32}\mathbf{.}\mathbf{00}\overline{)\mathbf{}\mathbf{g}\mathbf{}{\mathbf{O}}_{\mathbf{2}}\mathbf{}}}\mathbf{}\mathbf{=}\mathbf{}\mathbf{0}\mathbf{.}\mathbf{66}\mathbf{}\mathbf{mol}\mathbf{}{\mathbf{O}}_{\mathbf{2}}\mathbf{}\phantom{\rule{0ex}{0ex}}\mathbf{mol}\mathbf{}{\mathbf{N}}_{\mathbf{2}}\mathbf{}\mathbf{=}\mathbf{}\mathbf{79}\overline{)\mathbf{}\mathbf{g}\mathbf{}{\mathbf{N}}_{\mathbf{2}}\mathbf{}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{}{\mathbf{N}}_{\mathbf{2}}}{\mathbf{28}\mathbf{.}\mathbf{02}\overline{)\mathbf{}\mathbf{g}\mathbf{}{\mathbf{N}}_{\mathbf{2}}\mathbf{}}}\mathbf{=}\mathbf{}\mathbf{2}\mathbf{.}\mathbf{82}\mathbf{}\mathbf{mol}\mathbf{}{\mathbf{N}}_{\mathbf{2}}$

What is the mole fraction of oxygen gas in air (see table 5.3 in the textbook)?

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