Molecules of benzoic acid $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COOH}\right)$ dimerise in benzene. ' w ' g of the acid dissolved in 30 g of benzene shows a depression in freezing point equal to 2 K . If the percentage association of the acid to form dimer in the solution is 80 , then w is (Given that $\mathrm{K}_{\mathrm{f}}=5 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$, Molar mass of benzoic acid $=122 \mathrm{~g} \mathrm{~mol}^{-1}$ )