If compound AB dissociates...|LIQUID SOLUTION|JEE-Main 2021

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JEE MAIN 2021

25-02-2021 S2

Question

If a compound AB dissociates to the extent of $75 \%$ in an aqueous solution, the molality of the solution which shows a 2.5 K rise in the boiling point of the solution is $\_\_\_\_$ molal.
$$ \left[\mathrm{K}_{\mathrm{b}}=0.52 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right]
$$

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Solution

$$ \begin{aligned} & \alpha=0.75, n=2 \\ & i=1-\alpha+n \alpha=1-0.75+2 \times 0.75=1.75 \\ & \Delta T_b=i k_b m \end{aligned} $$ or, $2.5=1.75 \times 0.52 \times \mathrm{m}$
or, $\mathrm{m}=\frac{2.5}{1.75 \times 0.52}=2.74$
$\therefore \quad$ nearest integer answer will be 3

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