Density of dry air containing only N₂ and O₂ is 1.15 g/L at 740 mm of Hg and 300 K. What is % composition of N, by weight in the air ?

$\begin{array}{l}\mathrm{d}=\frac{\mathrm{PM}}{\mathrm{RT}}\\ 1.15=\frac{\left(\frac{740}{760}\right)\times \mathrm{M}}{0.0821\times 300}\end{array}$
$\therefore \mathrm{M}=29.09.$ Let mole fraction of N₂ is x.
$\begin{array}{l}29.09=28\times \mathrm{x}+32(1-\mathrm{x});\\ \mathrm{x}=0.7275\\ \mathrm{wt}. \mathrm{\%}= \mathrm{mole} \mathrm{\%}\times \frac{ \mathrm{mol}.\mathrm{wt}.\mathrm{of} {\mathrm{N}}_2}{ \mathrm{average} \mathrm{mol}.\mathrm{wt}.\mathrm{ofair} }\\ =72.75\times \frac{28}{29.09}=70.02\end{array}$