Question

Kjeldahl’s method is used for estimation of nitrogen. Nitrogen is quantitatively converted to ammonium sulfate. It is then treated with an alkali solution. The nitrogen present Is estimated as
A. ${N_2}$ gas
B. $N{O_2}$ gas
C. $N{H_3}$ gas
D. ${\left( {N{H_4}} \right)_2}S{O_4}$

Answer 1

The Kjeldahl method is a method that is used to determine the amount of nitrogen present in the given compound. It has three important steps that are digestion, distillation, and titration. It is used in analytical chemistry.

Complete answer:
We know that the Kjeldahl method is a very popular method for determining the amount of nitrogen present in any given compound. The process is carried out in three important steps that are digestion, distillation, and titration.
The first step is digestion that includes heating the given compound in a concentrated solution containing ${H_2}S{O_4}$ . The nitrogen in the compound reacts with sulfuric acid to form ammonium sulfate that is ${\left( {N{H_4}} \right)_2}S{O_4}$ . Various catalysts like ions of mercury and copper are used. We know that the substance has fully decomposed when it becomes colorless.
The equation is $\left[ N \right] + {H_2}S{O_4}\xrightarrow{{}}{\left( {N{H_4}} \right)_2}S{O_4}$
The second step is distillation, in this step we add an excess base like $NaOH$ to convert $N{H_4}$ in ${\left( {N{H_4}} \right)_2}S{O_4}$ to $N{H_3}$ . $N{H_3}$ is trapped in a solution of $HCl$ and water. The equation is as follow
${\left( {N{H_4}} \right)_2}S{O_4} + 2NaOH\xrightarrow{\Delta }N{a_2}S{O_4} + 2N{H_3} + 2{H_2}O$
In the third and final step, we do titration which is used to determine the amount of ammonia obtained or the amount of nitrogen present in the compound.

**So from here, we can see that the correct option is $C$

Note:**
The kjeldahl method does not apply to compounds containing azo groups. It is also not applicable for compounds containing nitro groups such as nitrate and nitrite. It only measures nitrogen bonded to the organic compound.