Question

In Dumas' method for estimation of nitrogen 1 g of an organic compound gave 150 mL of nitrogen collected at 300 K temperature and 900 mm Hg pressure. The percentage composition of nitrogen in the compound is _______ % (nearest integer). (Aqueous tension at $300 \mathrm{~K}=15 \mathrm{~mm} \mathrm{Hg}$ )

Hint:

Use the ideal gas law to calculate the moles of nitrogen and then determine the percentage composition.

Answer 1

Correct Answer: 20

1. Calculate the partial pressure of $\mathrm{N}_{2}$: \[ \mathrm{p}_{\mathrm{N}_2} = 900 \mathrm{~mm~Hg} - 15 \mathrm{~mm~Hg} = 885 \mathrm{~mm~Hg} \]
2. Calculate the moles of $\mathrm{N}_{2}$: \[ \text{Moles of } \mathrm{N}_2 = \frac{885 \mathrm{~mm~Hg} \times 0.15 \mathrm{~L}}{0.0821 \mathrm{~L~atm/mol} \times 300 \mathrm{~K}} = 0.0071 \mathrm{~mol} \]
3. Calculate the percentage of nitrogen: \[ \text{Percentage of nitrogen} = \frac{0.0071 \mathrm{~mol} \times 28 \mathrm{~g/mol}}{1 \mathrm{~g}} \times 100 = 19.85% \approx 20% \] Therefore, the correct answer is (20).