Benzoic acid associates in benzene to form dimers. The van't Hoff factor $i$ for benzoic acid in benzene (assuming complete dimerisation) is:

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.