1.0 g of coal sample was digested (Kjeldahl method). The liberated ammonia was absorbed in 25 ml of N/10 H2SO4. To neutralise excess acid, 15 ml of 0.1 N NaOH was required. Determine the % nitrogen in the given sample of coal.

Kjeldahl method: coal is digested with conc. H₂SO₄ + K₂SO₄ (with a CuSO₄ catalyst), which converts N to (NH₄)₂SO₄. Heating with NaOH liberates NH₃, which is absorbed in a known volume of standard acid. The unused acid is back-titrated with NaOH.

Given: W = 1.0 g; acid = 25 mL of 0.1 N; excess acid = 15 mL of 0.1 N NaOH.

Volume of 0.1 N acid used by NH₃ = 25 − 15 = 10 mL.

\( \%N = \dfrac{1.4 \times N \times V}{W} = \dfrac{1.4 \times 0.1 \times 10}{1.0} \)

% Nitrogen = 1.4 %