Q46 / 150 · MHT CET 2024 (03 May Shift 1) · Chemistry

Calculate the molar mass of nonvolatile solute when 1.5 g of it is dissolved in 90 g solvent decreases its freezing point by 0.25 K . $\left[\mathrm{K}_{\mathrm{f}}=1.2 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right]$

  • A
    $72 \mathrm{~g} \mathrm{~mol}^{-1}$
  • Correct answer
    B
    $80 \mathrm{~g} \mathrm{~mol}^{-1}$
  • C
    $88 \mathrm{~g} \mathrm{~mol}^{-1}$
  • D
    $96 \mathrm{~g} \mathrm{~mol}^{-1}$

Solution

$\begin{aligned} & \mathrm{M}_2=\frac{1000 \mathrm{~K}_{\mathrm{f}} \mathrm{W}_2}{\Delta \mathrm{~T}_{\mathrm{b}} \mathrm{W}_{\mathrm{i}}} \\ & =\frac{1000 \times 1.2 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1} \times 1.5 \mathrm{~g}}{0.25 \mathrm{~K} \times 90 \mathrm{~g}}=80 \mathrm{~g} \mathrm{~mol}^{-1}\end{aligned}$

Hence, the correct answer is option: 'B'.

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