Question

1.5 g of an organic compound was analysed by Kjeldahl's method for estimation of nitrogen. The ammonia liberated was passed into 30 mL of 1N HCl solution. The remaining HCl was further neutralised by 120 mL of N/10 NaOH solution. The percentage of nitrogen in the compound is:

• A. 18.6
• B. 16.8
• C. 36.3
• D. 28.4

Hint:

Percentage of Nitrogen = \(\frac{1.4 \times \text{Meq of acid consumed}}{\text{Mass of organic compound}}\).

Answer 1

The Correct Option is A

Step 1: Understand the Kjeldahl back-titration.
Nitrogen in the sample turns into ammonia, which is absorbed by a known excess of HCl. The unreacted HCl is then titrated with NaOH, so the difference tells us how much HCl the ammonia used.
Step 2: Find total milliequivalents of HCl.
Total HCl $= 30 \text{ mL} \times 1 \text{ N} = 30$ meq.
Step 3: Find HCl left over.
The back-titrating NaOH is $120 \text{ mL} \times 0.1 \text{ N} = 12$ meq, which equals the leftover HCl.
Step 4: Find HCl used by ammonia.
HCl that reacted with ammonia $= 30 - 12 = 18$ meq, and this equals the meq of nitrogen.
Step 5: Convert to mass of nitrogen.
Mass of N $= \dfrac{18 \times 14}{1000} = 0.252$ g.
Step 6: Find the percentage.
\[ \% N = \frac{0.252}{1.5} \times 100 \approx 18.6\% \] which is option 1.
\[ \boxed{18.6} \]