Solutions and Colligative Properties 2 Question 29
29. $1.22 \mathrm{~g} \mathrm{C}{6} \mathrm{H}{5} \mathrm{COOH}$ is added into two solvents and data of $\Delta T_{b}$ and $K_{b}$ are given as :
(i) In $100 \mathrm{~g} \mathrm{CH}{3} \mathrm{COCH}{3} \Delta T_{b}=0.17, K_{b}=1.7 \mathrm{~K} \mathrm{~kg} / \mathrm{mol}$
(ii) In $100 \mathrm{~g}$ benzene, $\Delta T_{b}=0.13$ and $K_{b}=2.6 \mathrm{~K} \mathrm{~kg} / \mathrm{mol}$
Find out the molecular weight of $\mathrm{C}{6} \mathrm{H}{5} \mathrm{COOH}$ in both the cases and interpret the result.
$(2004,2 \mathrm{M})$
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Solution:
- (i) $\Delta T_{b}=K_{b} \cdot m_{2}$
$$ \Rightarrow \quad 0.17=1.7 \times \frac{1.22}{M} \times \frac{1000}{100} \Rightarrow M=122 $$
(ii) $0.13=2.6 \times \frac{1.22}{M} \times \frac{1000}{100} \Rightarrow M=244$
The above molar masses suggests thapt benzoic acid is monomeric in acetone while dimeric in benzene.
