Question

Find the percentage of nitrogen in an organic compound analysed by Kjeldahl method $1.61$ g of the compound produced ${\rm{N}}{{\rm{H}}_{\rm{3}}}$ which was absorbed in ${\rm{250}}\;{\rm{mL}}$ of $\dfrac{{\rm{N}}}{{\rm{2}}}\;{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}$ solution. The remaining acid was then diluted to one litre, ${\rm{25}}\;{\rm{mL}}$ of which required ${\rm{25}}{\rm{.5}}\;{\rm{mL}}$ of ${\rm{N/10}}$ NaOH for exact neutralization.

Answer 1

Hint:: We know that the Kjeldahl method is generally used for the determination of nitrogen percentage in any given organic compound by several chemical equations. The volumes of acid and base are needed for the Kjeldahl method.

Complete answer:
The given mass of the organic compound is ${\rm{1}}{\rm{.61}}\;{\rm{g}}$.
The given volume of sulfuric acid is ${\rm{250}}\;{\rm{mL}}$.
The given volume of sodium hydroxide solution is ${\rm{25}}{\rm{.5}}\;{\rm{mL}}$.
Now, let us first calculate the percentage of nitrogen by Kjeldahl’s method by using the formula as shown below.
${\rm{\% }}\;{\rm{of}}\;{{\rm{N}}_{\rm{2}}}\;{\rm{by}}\;{\rm{Kjeldlal's}}\;{\rm{method}} = \dfrac{{1.4 \times M \times 2 \times \left( {\dfrac{{V - {V_1}}}{2}} \right)}}{{{\rm{Mass}}\;{\rm{of}}\;{\rm{organic}}\;{\rm{compound}}}}$
Where,
$M$ is the molarity of the substance.
$V$ is the volume of sulfuric acid.
${V_1}$ is the volume of sodium hydroxide solution.
The value of $\dfrac{{\rm{N}}}{{\rm{2}}}\;{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} = \dfrac{{\rm{M}}}{4}\;{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}$ which means ${\rm{n}} = 2$.
Substitute the respective values in the above equation. We get,
$\begin{array}{c}
{\rm{\% }}\;{\rm{of}}\;{{\rm{N}}_{\rm{2}}}\;{\rm{by}}\;{\rm{Kjeldlal's}}\;{\rm{method}} = \dfrac{{1.4 \times \dfrac{1}{4} \times 2 \times \left( {\dfrac{{250 - 25.5}}{2}} \right)}}{{{\rm{1}}{\rm{.61}}\;{\rm{g}}}}\\
 = 48.80\%
\end{array}$

Therefore, the percentage of nitrogen in an organic compound analysis by Kjeldahl method is $48.80\% $.

Note::: The Kjeldahl method is used for the quantitative determination of nitrogen containing compounds that is protein, amines and several organic compounds. It is used to evaluate the quantity of protein in the beer.