Question

During nitrogen estimation of an organic compound by Kjeldahl's method, the ammonia evolved by 1 g of the compound neutralised 20 mL of 0.5 M $H_2SO_4$. Calculate the percentage of nitrogen in the compound.

Answer 1

28%

Answer 2

1. Moles of $H_2SO_4$:

Volume = 20 mL = 0.02 L, Molarity = 0.5 M
Moles of $H_2SO_4$ = 0.02 × 0.5 = 0.01 moles

2. Moles of $NH_3$ evolved:

Reaction: $H_2SO_4 + 2NH_3 \rightarrow (NH_4)_2SO_4$
For every 1 mole of $H_2SO_4$, 2 moles of $NH_3$ are required.
Moles of $NH_3$ = 2 × 0.01 = 0.02 moles

3. Grams of Nitrogen:

Each mole of $NH_3$ carries 1 mole of nitrogen (atomic mass = 14 g).
Grams of nitrogen = 0.02 × 14 = 0.28 g

4. Percentage of Nitrogen in the Compound:

Since 1 g of the compound produced 0.28 g of nitrogen,
Percentage N = (0.28/1) × 100 = 28%