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. (Rounded off to the nearest integer)
[Kb = 0.52 K kg mol$-$1]
Answer (integer)
3
Solution
<p>As AB is a binary electrolyte,</p>
<p>$\therefore$ AB $\rightleftharpoons$ A<sup>+</sup> + B<sup>$-$</sup>, n = 2</p>
<p>$i = 1 + \alpha (n - 1) = 1 + {{75} \over {100}}(2 - 1) = 1.75$</p>
<p>Given, $\Delta$T<sub>b</sub> = 2.5 K</p>
<p>K<sub>b</sub> = 0.52 K kg mol<sup>$-$1</sup></p>
<p>$\therefore$ $\Delta$T<sub>b</sub> = K<sub>b</sub> $\times$ m $\times$ i</p>
<p>$$ \Rightarrow m = {{\Delta {T_b}} \over {{K_b} \times i}} = {{2.5} \over {0.52 \times 1.75}}$$</p>
<p>$= 2.74 \simeq 3$ mol/kg</p>
About this question
Subject: Chemistry · Chapter: States of Matter · Topic: Gas Laws
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