Addition of 0.223g of phenyl acetic acid (C₆H₅CH₂COOH) to 4.4g of benzene decreases the freezing point of benzene by 0.83°C. It shows (k_f = 5.12 k kg mol^–1)

$$\begin{gathered} \mathrm{Molar} \mathrm{mass} \mathrm{of} \mathrm{solute}(\mathrm{phenyl} \mathrm{acetic} \mathrm{acid})=136\mathrm{g} \\ {\mathrm{W}}_{\mathrm{s}}=0.223\mathrm{g} ; {\mathrm{W}}_{\mathrm{o}}=4.4\mathrm{g} ; ∆{\mathrm{T}}_{\mathrm{f}}=0.83°\mathrm{c} \\ ∆{\mathrm{T}}_{\mathrm{f}}=\mathrm{i}\times {\mathrm{K}}_{\mathrm{f}}\times \frac{{\mathrm{W}}_{\mathrm{s}}}{\mathrm{Molar} \mathrm{mass}}\times \frac{1000}{{\mathrm{W}}_{\mathrm{o}}} \\ 0.83=\mathrm{i}\times 5.12\times \frac{0.223}{136}\times \frac{1000}{4.4} \\ \mathrm{i}=0.435<1 \end{gathered}$$

Therefore, Phenyl acetic acid associates, i.e. dimerises in Benzene.