CBSE 2023 · Region 3 · Set 1 · Q28 · 3 marks
0.$\displaystyle 3$ g of acetic acid $\displaystyle \left(\mathrm{M}=60 \mathrm{~g} \mathrm{~mol}^{-1}\right)$ dissolved in $\displaystyle 30$ g of benzene shows a depression in freezing point equal to $\displaystyle 0.45^{\circ} \mathrm{C}$. Calculate the percentage association of acid if it forms a dimer in the solution. (Given : $\displaystyle \mathrm{K}_{\mathrm{f}}$ for benzene $\displaystyle =5 \cdot 12 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$ )
Marking-scheme solution
\[\begin{aligned}
& \Delta \mathrm{T}_{\mathrm{f}}=\mathrm{i} \mathrm{~K}_{\mathrm{f}} \frac{\mathrm{~W}_{\mathrm{B}}}{\mathrm{M}_{\mathrm{B}}} \times \frac{1000}{\mathrm{~W}_{\mathrm{A}}} \\
& \cdot 45=\mathrm{i} \times 5 \cdot 12 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1} \times \frac{0 \cdot 3 \mathrm{~g}}{60 \mathrm{~g} \mathrm{~mol}^{-1}} \times \frac{1000}{30 \mathrm{~kg}} \\
& \mathrm{i}=0 \cdot 527 \\
& \alpha=\frac{1-\mathrm{i}}{1-\dfrac{1}{n}} \\
& \alpha=\frac{1-0 \cdot 527}{1-\dfrac{1}{2}}=0 \cdot 946 \text { or } 94 \cdot 6 \%
\end{aligned}
\]
Asked in 2 different years
- 2023 · Region 3 · Set 1 · Q28
- 2023 · Region 3 · Set 2 · Q29
- 2023 · Region 3 · Set 3 · Q29
- 2025 · Region 7 · Set 1 · Q22
- 2025 · Region 7 · Set 2 · Q28
- 2025 · Region 7 · Set 3 · Q23
SolutionsColligative Properties and Determination of Molar MassApplyshort_answerhard
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Asked in 2023 & 2025. CBSE Class 12 Chemistry past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.