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Benzoic acid associates in benzene to form dimers. The van't Hoff factor $i$ for benzoic acid in benzene (assuming complete dimerisation) is:
A$2.0$
B$0.5$
C$1.0$
D$1.5$
Answer & Solution
Correct answer: B. $0.5$
For dimerisation $2 \mathrm{A} \to \mathrm{A_2}$, two molecules combine into one. So the number of effective particles is halved relative to undissociated solute and $i = 0.5$. This is why observed colligative properties of benzoic acid in benzene give an apparent molar mass roughly twice the true value.
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