Nitrogen can be estimated by Kjeldahl's method for which of the following compound?

Solution

Chemistry ma’am · 3 steps

Kjeldahl works only if the nitrogen can become ammonium sulphate, so look for a plain amine nitrogen.

1. How Kjeldahl works. The compound is heated with concentrated H₂SO₄, which turns its nitrogen into (NH₄)₂SO₄. Alkali then releases NH₃, which is absorbed in a known amount of acid and measured by titration.
2. Where it fails. Any nitrogen that does not end up as (NH₄)₂SO₄ is not counted. This happens with nitro and azo groups, with diazonium salts, and with nitrogen that is part of a ring such as pyridine.

   Option	Compound	Nitrogen present as	Kjeldahl works?
   (a)	Nitrobenzene, C₆H₅NO₂	nitro group	No, not fully converted to (NH₄)₂SO₄
   (b)	Pyridine, C₅H₅N	nitrogen inside the ring	No, the ring nitrogen resists digestion
   (c)	Benzenediazonium chloride, C₆H₅N₂⁺Cl⁻	diazo group	No, nitrogen escapes as N₂ gas on heating
   (d)	Benzylamine, C₆H₅CH₂NH₂	-NH₂ on a side chain	Yes

3. So. Only benzylamine has its nitrogen as a simple amino group, which is fully converted to (NH₄)₂SO₄. The answer is (d).

The idea to keep. Kjeldahl fails for nitro and azo groups, diazonium salts and ring nitrogen like pyridine. Amines and amides are safe, because their nitrogen turns completely into (NH₄)₂SO₄.

Answer from the NTA final answer key. Solution by PrepWiser.