JEE Main · 2010 · 11 Apr 2010 · Offline · Practical and Analytical Chemistry

In the Kjeldahl method the ammonia set free is caught in a known amount of acid, and the leftover acid is found by back-titration. The acid taken was $20\times 0.1=2.0$ mmol and the NaOH needed was $15\times 0.1=1.5$ mmol, so the ammonia neutralised $2.0-1.5=0.5$ mmol of acid. That is $0.5$ mmol of nitrogen, a mass of $0.5\times 10^{-3}\times 14=7\times 10^{-3}$ g. As a fraction of the $29.5$ mg sample this is $\frac{7\times 10^{-3}}{29.5\times 10^{-3}}\times 100\approx 23.7\%$.