Question

Air is about 78% nitrogen gas (N2) by mass. The molar mass of nitrogen gas is 28.0 g/mol. A 100.0-g sample of air contains how many moles of nitrogen?

Answer 1

\[100.0\text{ g air}~~\implies~~\sim78\text{ g N}_2\]
\[78\text{ g}\times\frac{1\text{ mol}}{28.0\text{ g}}=\cdots\text{ mol}\]