If a compound $\mathrm{AB}$ dissociates to the extent of $75 \%$ in an aqueous solution, the molality of the solution which shows a $2.5 \mathrm{~K}$ rise in the boiling point of the solution is____________ molal. (Rounded-off to the nearest integer)
$\left[\mathrm{K}_{\mathrm{b}}=0.52 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right]$
$\alpha=0.75, \mathrm{n}=2$
$\mathrm{i}=1-\alpha+\mathrm{n} \alpha=1-0.75+2 \times 0.75=1.75$
$\Delta \mathrm{T}_{\mathrm{b}}=\mathrm{ik}_{\mathrm{b}} \mathrm{m}$
or, $2.5=1.75 \times 0.52 \times \mathrm{m}$
or, $\mathrm{m}=\frac{2.5}{1.75 \times 0.52}=2.74$
$\therefore$ nearest integer answer will be 3
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