The freezing point of benzene decreases by ${0.45}^{°}\mathrm{C}$ on adding $0.2\mathrm{ }\mathrm{g}$ of acetic acid to $20 \mathrm{g}$ of benzene. If acetic acid associates to form a dimer in benzene, then what is the percentage association of acetic acid in benzene?

$\left({\mathrm{K}}_{\mathrm{f}} \mathrm{for}\mathrm{ }\mathrm{benzene}=5.12\mathrm{ }\mathrm{K}\mathrm{ }\mathrm{kg}\mathrm{ }{\mathrm{mol}}^{-1}\mathrm{ }\right)$