Question

Kjeldahl's method can be used for:

• A. $PhN{O}_2$
• B. $PhNHCOPh$
• C. Azobenzene
• D. Pyridine

Answer 1

The correct option is B $PhNHCOPh$

Kjeldahl's method is a method for the quantitive determination of nitrogen contained in organic substances as well as nitrogen contained in inorganic compounds like ammonia and ammonium salts.
But, Kjeldahl's method cannot be used for nitrogen determination of compounds having nitro group or azo-group or nitrogen present in rings as the nitrogen of these compounds cannot be converted to ammonium sulphate $(N{H}_4{)}_{2}S{O}_4$ under the given conditions in this method. Hence, (b) is correct.
The reaction is :
$\text{Organic compound}+H_{2}S{O}_4\to (N{H}_4{)}_{2}S{O}_4$