$20 \%$ of acetic acid is dissociated when its $5 \mathrm{~g}$ is added to $500 \mathrm{~mL}$ of water. The depression in freezing point of such water is _________ $\times 10^{-3}{ }^{\circ} \mathrm{C}$.

Atomic mass of $\mathrm{C}, \mathrm{H}$ and $\mathrm{O}$ are 12,1 and 16 a.m.u. respectively.

[Given : Molal depression constant and density of water are $1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$ and $1 \mathrm{~g} \mathrm{~cm}^{-3}$ respectively.]