The freezing point of a solution containing 0.2g of acetic acid in 20.0g benzene is lowered by $0.45°\mathrm{C}$. The degree of association of acetic acid in benzene is ________ (nearest integer)
(Assume acetic acid dimerises in benzene and ${\mathrm{K}}_{\mathrm{f}}$ for benzene =5.12 $\mathrm{K} \mathrm{kg}{ \mathrm{mol}}^{-1}$)
${\mathrm{M}}_{\text{observed }}$ of acetic acid =113.78