Question:
The molarity of $\mathrm{HNO}_{3}$ in a sample which has density $1.4 \mathrm{~g} / \mathrm{mL}$ and mass percentage of $63 \%$ is_________. (Molecular Weight of $\mathrm{HNO}_{3}=63$ )
Solution:
(14.00)
Mass percent of $\mathrm{HNO}_{3}=63$
Thus, $100 \mathrm{~g}$ of nitric acid solution contains $63 \mathrm{~g}$ of nitric acid by mass.
No. of moles $=\frac{63 \mathrm{~g}}{63 \mathrm{~g} \mathrm{~mol}^{-1}}=1$
Volume of $100 \mathrm{~g}$ of nitric acid solution
$=\frac{\text { Mass }}{\text { Density }}=\frac{100 \mathrm{~g}}{1.4 \mathrm{~g} / \mathrm{mL}}=71.4 \mathrm{~mL}$
Molarity $=\frac{\text { No. of moles }}{\text { volume }(\mathrm{mL})} \times 1000$
=$\frac{1}{71.4} \times 1000=14 \mathrm{M}$