Q48 / 150 · MHT CET 2025 (21 Apr Shift 1) · Chemistry
Calculate molality of the solution containing nonvolatile solute if boiling point elevation of the solution is $0.2 \mathrm{~K}\left[\mathrm{~K}_{\mathrm{b}}=0.52 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right]$
- A$0.162 \mathrm{~mol} \mathrm{~kg}^{-1}$
- B$0.281 \mathrm{~mol} \mathrm{~kg}{ }^{-1}$
- Correct answerC$0.385 \mathrm{~mol} \mathrm{~kg}^{-1}$
- D$0.501 \mathrm{~mol} \mathrm{~kg}^{-1}$
Solution
The boiling point elevation $\Delta T_b = K_b m$ relates to molality $m$ through the ebullioscopic constant $K_b = 0.52 \mathrm{K\ kg\ mol^{-1}}$.
Given $\Delta T_b = 0.2 \mathrm{K}$, solving for molality yields $m = \frac{\Delta T_b}{K_b} = \frac{0.2}{0.52} = 0.385 \mathrm{mol\ kg^{-1}}$ to three significant figures.
This value corresponds to option C among the given choices.
Hence, the correct answer is option: 'C'.