Question

For the estimation of nitrogen, ${\text{1}}{\text{.4g}}$ of organic compound was digested by Kjeldahl method and the evolved ammonia was absorbed in \[60ml\] of \[\dfrac{M}{{10}}\] sulphuric acid. The unreacted acid required \[20ml\] of \[\dfrac{M}{{10}}\] sodium hydroxide for complete neutralization. The percentage of nitrogen in the compound is:
a.) $50\% $
b.) $20\% $
c.) $60\% $
d.) $10\% $

Answer 1

Hint: Kjeldahl method- This is also known as Kjeldahl digestion. This is the method used for the quantitative determination of nitrogen contained in organic substances and also this method is used in the quantitative determination of the nitrogen contained in the inorganic ammonia and ammonium.

Complete step by step solution:
 Mass of the organic compound $ = m = 1.4g$
Here it is given that the compound is going under the Kjeldahl method and the ammonia evolved was absorbed by the sulphuric acid.
So, the milli equivalent of sulphuric acid = $60ml \times \dfrac{M}{{10}} \times 2 = 12$ milli equivalent.
It is also given that the unreacted amount of acid is neutralized by the sodium hydroxide.
So, the milliequivalent of the sodium hydroxide= $20ml \times \dfrac{M}{{10}} \times 1 = 2$ milli equivalent.
Amount of equivalent left to react with ammonia= $12 - 2 = 10$ milli equivalent of the sulphuric acid.
And the chemical reaction related to the above condition,
$2N{H_3} + {H_2}S{O_4} \to {(N{H_4})_2}S{O_4}$
From the above reaction we conclude that the $2$mole of ammonia requires $1$mole of ${H_2}S{O_4}$.
This implies that the \[10\] milli equivalent of sulphuric acid ${H_2}S{O_4}$will react with $20$milli equivalent of ammonia.
Now we will calculate the weight of nitrogen in the given organic compound.
So, the weight of the nitrogen $N$=$\dfrac{{{{(equivalent)}_{ammonia}} \times {M_{{N_2}}}}}{m}$
Where, ${M_{{N_2}}}$=atomic mass of the nitrogen.
So, the weight of the nitrogen $N$= $\dfrac{{\dfrac{{20}}{{1000}} \times 14}}{{1.4}}$
$ = \dfrac{{20 \times 14}}{{1000 \times 1.4}}$

$ = \dfrac{2}{{10}}$ , this is the weight of the nitrogen but we have to find the percentage weight of the nitrogen.
So, the percentage weight of the nitrogen in the given organic compound=$[\dfrac{2}{{10}} \times 100]\% = 20\% $
Hence the percentage of nitrogen in the compound is $20\% $.
So, the option (B) is the correct answer.

Note: The Kjeldahl method was developed by a brewer called Johann Kjeldahl in 1883. This was developed on the basis of the principle that the strong acid helps in the digestion of food so that it releases nitrogen which can be determined by a suitable titration technique. Hence in simple words, the Kjeldahl method is used to convert protein to ammonia by using the hot acid, which is then distilled into a standardized acid and the acid is back titrated to give the quantitative estimate. In this process a catalyst is also required to convert the organic nitrogen into the ammonium sulphate.