$20 \%$ of acetic acid is dissociated when its 5 g is added to 500 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 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.