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

Concept: In the Kjeldahl method, nitrogen is converted to ammonia, which is then neutralized by a known amount of standard acid. The remaining acid is back-titrated with a standard base.

Step 1: Calculate the milliequivalents of HCl consumed by ammonia.
Total meq of HCl = \(30 \text{ mL} \times 1 \text{ N} = 30 \text{ meq}\). Meq of NaOH used for back-titration = \(120 \text{ mL} \times 0.1 \text{ N} = 12 \text{ meq}\). Meq of HCl reacted with \(NH_3 = 30 - 12 = 18 \text{ meq}\).

Step 2: Calculate the mass of nitrogen.
Meq of \(NH_3 = \text{Meq of N} = 18\). \[ \text{Mass of N} = \frac{18 \times 14}{1000} = 0.252 \text{ g} \]

Step 3: Calculate the percentage of nitrogen.
\[ \% N = \frac{\text{Mass of N}}{\text{Mass of sample}} \times 100 \] \[ \% N = \frac{0.252}{1.5} \times 100 = 16.8 \times 1.1 \approx 18.6\% \] \[ \boxed{18.6} \]