1.2 g of a coal sample was digested by Kjeldahl's method. The ammonia liberated was absorbed in 40 mL of 0.1 N H₂SO₄. The excess acid required 18 mL of 0.1 N NaOH for neutralisation. Calculate the percentage of nitrogen in the coal.

Answer: Nitrogen \(=2.57\%\).

Principle: the coal is heated with concentrated H₂SO₄ (with K₂SO₄ and CuSO₄ as catalyst), which converts its nitrogen into (NH₄)₂SO₄. Excess NaOH then liberates NH₃, which is absorbed in a known volume of standard acid. The unused acid is back-titrated with standard NaOH.

Volume of acid used by NH₃ (both solutions are 0.1 N):

\(=40-18=22\) mL of 0.1 N H₂SO₄.

Formula:

\[\%N=\frac{\text{Volume of acid used}\times\text{Normality}\times1.4}{\text{Weight of coal}}=\frac{22\times0.1\times1.4}{1.2}\]

\[\%N=\frac{3.08}{1.2}=2.57\%\]

(Where 1.4 comes from: 1000 mL of 1 N acid ≡ 14 g N, so 1 mL of 1 N acid ≡ 0.014 g N; multiplied by 100 for percentage.)