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If sodium sulphate is considered to be completely dissociated into cations and anions in aqueous solution, the change in freezing point of water (ΔTf), when 0.01 mol of sodium sulphate is dissolved in 1 kg of water, is (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 i = 3. dTf = i*Kf*m = 3 * 1.86 * 0.01 = 0.0558 K.

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