In a Kjeldahl estimation, of an organic compound was digested, and the evolved ammonia was absorbed — Qualitative and Quantitative Analysis Chemistry Question
Question
In a Kjeldahl estimation, $29.5 \text{ mg}$ of an organic compound was digested, and the evolved ammonia was absorbed in $20 \text{ mL}$ of $0.1 \text{ M}$ $HCl$. The excess acid required $15 \text{ mL}$ of $0.1 \text{ M}$ $NaOH$ for complete neutralization. What is the percentage of nitrogen in the compound?
Answer: B
💡 Solution & Explanation
Milli-moles of $HCl$ taken = $20 \times 0.1 = 2 \text{ mmol}$. Milli-moles of $NaOH$ used = $15 \times 0.1 = 1.5 \text{ mmol}$. Milli-moles of $HCl$ neutralized by $NH_3$ = $2 - 1.5 = 0.5 \text{ mmol}$. Moles of $NH_3$ = $0.5 \times 10^{-3} \text{ mol}$. Mass of N = $0.5 \times 10^{-3} \times 14 = 7 \text{ mg}$. $\%N = (7 / 29.5) \times 100 \approx 23.7\%$.
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