In Dumas' method for estimation of nitrogen, 0.4 g of an organic compound gave 60 mL of nitrogen collected at 300 K…

🧠 Subtract the water vapour first In Dumas' method the gas is collected over water. So the $715\,\text{mm}$ reading is $\ce{N2}$ pressure plus water vapour pressure (aqueous tension). Subtract the tension to get pure $\ce{N2}$ pressure.

🗺️ Step by step Dry pressure of $\ce{N2}$: $P = 715 - 15 = 700\,\text{mm Hg} = \frac{700}{760}\,\text{atm}.$

Use $PV = nRT$ with $V = 0.060\,\text{L}$, $T = 300\,\text{K}$, $R = 0.0821$: $n = \frac{(700/760)(0.060)}{0.0821 \times 300} \approx 2.244 \times 10^{-3}\,\text{mol}.$

Mass of N $= n \times 28 \approx 0.0628\,\text{g}$.

$\%\,\text{N} = \frac{0.0628}{0.40} \times 100 \approx 15.71\%.$

⚠️ Common mistake Skipping the aqueous tension correction is the most common error. You will get a slightly higher $\%$N (around $16.05$) — close enough to fool you, but wrong.

$\boxed{\text{Answer: (a)}}$