Question

By dissolving $ 5\,\,g $ substance in $ 50\,\,g $ of water, the decrease in freezing point is $1.2^{\circ} C $ . The gram molal depression is $ 1.85^{\circ} C $ . The molecular weight of substance is:

• A. 105.4
• B. 118.2
• C. 137.2
• D. 154.2

Answer 1

Answer: (D)

We know that $m=\frac{1000 K_{f} \times w}{\Delta T W}$
Here, $ \Delta T =1.2^{\circ} C , K_{f}=1.85^{\circ}$
$ w =5 \,g, W=50 \,g$
$\therefore m =\frac{1000 \times 1.85 \times 5}{1.2 \times 50}=154.2$