The freezing point of benzene decreases by $0.45^{\circ}C$ when $0.2\; g$ of acetic acid is added to $20 \;g$ of benzene. If acetic acid associates to form a dimer in benzene, percentage association of acetic acid in benzene will be : $(K_f$ for benzene = $5.12\; K \;kg\; mol^{–1} )$
