Q.28. Find the depression in freezing point of solution when 3.2 gram non volatile solute with molar mass 128 \mathrm{gram} \mathrm{mol}^{-1} is dissolved in 80 gram solvent if cryoscopic constant of solvent is 4.8 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}.
A. 3.0 K
B. 1.5 K
C. 2.0 K
D. 2.5 K
Answer: B. 1.5 K
Explanation :-
\Delta \mathrm{T}_{\mathrm{f}} = \frac{1000 \mathrm{~K}_{\mathrm{f}} \mathrm{~W}_2}{\mathrm{M}_2 \mathrm{~W}_1} = \frac{1000 \mathrm{~g} \mathrm{~kg}^{-1} \times 4.8 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1} \times 3.2 \mathrm{~g}}{128 \mathrm{~g} \mathrm{~mol}^{-1} \times 80 \mathrm{~g}} = 1.5 \mathrm{~K}