Question

For the estimation of nitrogen, $1.4{\text{g}}$ of an organic compound was digested by Kjeldahl method and evolved ammonia was absorbed in $60{\text{mL}}$ of $\dfrac{M}{{10}}$ sulphuric acid. The unreacted acid required $20{\text{mL}}$ of $\dfrac{M}{{10}}$ sodium hydroxide for complete neutralization. The percentage of nitrogen in the compound is:

Answer 1

To answer this question, you must recall the methods of quantitative and qualitative analysis of organic compounds. The Kjeldahl method is used for calculation the amount of nitrogen present in a given organic compound.
Formula used:
$\% {\text{N}} = \dfrac{{1.4{\text{NV}}}}{{\text{W}}}$
Where, N is the normality of acid required to neutralize the ammonia evolved
V is the volume of ammonia
W is the weight of the given substance
[Also, here ${\text{N*V}}$ gives the amount of ammonia in the compound.]

Complete step by step answer:
The principle of the Kjeldahl method is that when we heat an organic compound containing nitrogen with concentrated sulphuric acid, the nitrogen is converted into ammonium sulphate.
When we treat the resulting solution with excess of sodium hydroxide solution, we get ammonia gas. Ammonia is then absorbed in excess of standard sulphuric acid.
Then we determine the amount of ammonia and thus the amount of nitrogen by finding the amount of acid neutralized by the back titration of the residual acid with a standard alkali solution. From this we can calculate the percentage of nitrogen.
We are given in the question that,
The weight of the organic compound$= W = 1.4g$
Given,
$60{\text{mL}}$of $\dfrac{M}{{10}}$sulphuric acid$= \dfrac{1}{5} \times 60 = 12{\text{Meq}}$.
$20{\text{mL}}$of $\dfrac{M}{{10}}$ sodium hydroxide$= \dfrac{1}{{10}} \times 20 = 2{\text{Meq}}$.
Hence, sulphuric acid used against ammonia$= 12 - 2 = 10{\text{Meq}}$
Therefore, we can write the percentage of nitrogen as,
$\% {\text{N}} = \dfrac{{1.4 \times 10}}{{1.4}}$
Solving this, we get:
$\therefore \% {\text{N}} = 10\%$

Thus, the percentage of nitrogen in the compound is $10\%$.

Note:
The Kjeldahl method is not useful for those nitrogen-containing organic compounds which produce nitrogen gas on heating. Some examples of such compounds are, nitro compounds, azo compounds, compounds containing nitrogen in ring, etc. For the quantitative analysis of nitrogen in such compounds, we can use the Duma’s method.