Question

A sample of 0.50 g of an organic compound was treated according to Kjeldahl’s method. The ammonia evolved was absorbed in 50 mL of 0.5 M $H_{2}S{O}_4$. The residual acid required 60 mL of 0.5 M solution of NaOH for neutralisation. Find the percentage composition of nitrogen in the compound.

Answer 1

Given that, total mass of organic compound = 0.50 g
60 mL of 0.5 M solution of NaOH was required by residual acid for neutralisation.
60 ml of 0.5 M NaOH solution $=\frac{60}{2}$ mL of 0.5 M~h_2SO_4 = 30~mL\) of 0.5 M $H_{2}S{O}_4$
$\therefore$ Acid consumed in absorption of evolved ammonia is (50–30) mL = 20 mL
Again, 20 mL of 0.5 $M{H}_{2}S{O}_4=40mLof0.5MN{H}_3$
Also, since 1000 mL of 1 $MN{H}_3$ contains 14 g of nitrogen,
$\therefore$ 40 mL of 0.5 M $N{H}_3$ will contain $\frac{14\times 40}{1000}\times 0.5=0.28gofN$
Therefore, percentage of nitrogen in 0.50 g of organic compound $=\frac{0.28}{0.50}\times 100=56\%$