JEE Main 2019 · Chemistry · Qualitative and Practical Chemistry · medium

In a Kjeldahl determination, 0.300 g of an organic compound releases all its nitrogen as NH3, which is absorbed in 30.0 mL of 0.200 M H2SO4. The…

Single correct option

In a Kjeldahl determination,

0.300\ \mathrm{g}

of an organic compound releases all its nitrogen as

\mathrm{NH_3}

, which is absorbed in

30.0\ \mathrm{mL}

0.200\ \mathrm{M}\ \mathrm{H_2SO_4}

. The excess acid needs exactly

20.0\ \mathrm{mL}

0.400\ \mathrm{M}

NaOH. The percentage of nitrogen in the compound is nearest to (N = 14)

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Answer: (B) $18.7$

Total acid

\mathrm{H^+} = 30.0\times0.200\times2 = 12.0

mmol. NaOH used

=20.0\times0.400 = 8.0

mmol, which measures the excess

\mathrm{H^+}

. So

\mathrm{H^+}

consumed by

\mathrm{NH_3}

12.0-8.0 = 4.0

mmol, equal to

4.0

mmol N

=56

mg. Percentage

=56/300\times100 = 18.7\%

. Forgetting that

\mathrm{H_2SO_4}

is dibasic gives

9.3

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