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 ______ $$\times 10^{-3}°C$$. Atomic mass of C, H and O are 12, 1 and 16 a.m.u. respectively.
[Given : Molal depression constant and density of water are $$1.86 \text{ K kg mol}^{-1}$$ and $$1 \text{ g cm}^{-3}$$ respectively.