Question

Dry air drawn thrugh bulbs containing a solution of 40 grams of urea in 300 grams of water, then through bulbs containing pure water at the same temperature and finally through a tube in which pumice moistened with strong $H_2$$S{O}_4$ was kept. The water bulbs lost 0.0870 grams and the sulphuric acid tube gained 2.036 grams. Calculate the molecular weight of urea.

• A. $M=23.4$
• B. $M=53.8$
• C. $M=64.5$
• D. $M=107.6$

Answer 1

The correct option is B $M=53.8$

Loss in weight of solution is proportional to $P_A$
Loss in weight of solvent is proportional to $P_A^0-P_A$
Hence, $\frac{P_A^0-P_A}{P_A}=\frac{0.0870}{2.036-0.0870}=0.0446$
According to Raoult's law $\frac{P_A^0-P_A}{P_A}=\frac{wM}{Wm}=\frac{40\times 18}{300\times m}$
Hence, $m=53.8$