Question

Calculate the depression in freezing point of a solution when $4\,\text{g}$ non-volatile solute of molar mass $126\,\text{g mol}^{-1}$ is dissolved in $80\,\text{mL}$ water. $[K_f = 1.86\ \mathrm{K\,kg\,mol^{-1}}]$

• A. 0.55 K
• B. 0.74 K
• C. 0.86 K
• D. 0.90 K

Hint:

Chemistry Tip: For water, 1 mL $\approx$ 1 g in dilute solutions.

Answer 1

The Correct Option is B

Step 1: Use formula: $$ \Delta T_f=K_f m $$

Step 2: Moles of solute: $$ =\frac{4}{126}=0.03175 $$

Step 3: Mass of water $=80$ mL $=80$ g $=0.08$ kg

Step 4: Molality: $$ m=\frac{0.03175}{0.08}=0.3969 $$

Step 5: Calculate: $$ \Delta T_f=1.86\times0.3969=0.74\text{ K} $$ $$ \therefore \text{Correct option is (B).} $$