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Assuming sodium sulphate undergoes complete dissociation into its ions in water, what is the depression in the freezing point of water when 0.01 mol of sodium sulphate is dissolved in 1 kg of water? (Given: Kf = 1.86 K kg mol−1)

1. 0.372 K
2. 0.0558 K
3. 0.0744 K
4. 0.0186 K

Correct answer: 0.0558 K

Solution

Na2SO4 -> 2Na+ + SO4^2-, so van't Hoff factor i=3. dTf = i*Kf*m = 3*1.86*0.01 = 0.0558 K.

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