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A certain substance 'A' tetramerises in water to the extent of . A solution of of A in of water loweSolutions and Colligative Properties Chemistry Question

Question

A certain substance 'A' tetramerises in water to the extent of $80\%$. A solution of $2.5\text{ g}$ of A in $100\text{ g}$ of water lowers the freezing point by $0.3^\circ\text{C}$. The molar mass of the pure monomer A is: ($K_f = 1.86$)

Answer: D

💡 Solution & Explanation

Tetramerisation means $n=4$. $\alpha = 0.8$. $i = 1 + 0.8(\frac{1}{4} - 1) = 1 + 0.8(-0.75) = 1 - 0.6 = 0.4$. $\Delta T_f = i \times K_f \times m \implies 0.3 = 0.4 \times 1.86 \times \frac{2.5/M}{0.1}$. Solving for $M$: $M = \frac{0.4 \times 1.86 \times 25}{0.3} = 62\text{ g mol}^{-1}$.

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