The freezing point of benzene decreases by  ${0.45}^{0}C$ when 0.2g of acetic acid is added to 20g of benzene. If acetic acid associataed to form a dimer in benzene, percentage of association of acetic acid in benzene will be ($K_f$ for benzene=5.12 K $Kgmo{l}^{-1}$ )