JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Practical and Analytical Chemistry

The nitrogen is collected over water, so the dry-gas pressure is $715-15=700$ mm Hg, that is $\frac{700}{760}$ atm. The moles of nitrogen gas are $n=\frac{PV}{RT}=\frac{(700/760)(0.050)}{(0.0821)(300)}=1.87\times 10^{-3}$ mol. Each molecule holds two nitrogen atoms, so the mass of nitrogen is $2\times 1.87\times 10^{-3}\times 14=0.0524$ g, i.e. $52.4$ mg. Expressed as a fraction of the $292$ mg sample, $\frac{52.4}{292}\times 100=17.9\%$, which to the nearest integer is $18\%$.