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A 0.2 molal aqueous solution of the weak acid HX has a degree of dissociation of 0.3. If the cryoscopic constant of water is 1.85, the freezing point of this solution is closest to

1. −0.360°C
2. −0.260°C
3. +0.480°C
4. −0.480°C

Correct answer: −0.480°C

Solution

i = 1 + alpha = 1 + 0.3 = 1.3. dTf = i*Kf*m = 1.3 * 1.85 * 0.2 = 0.481. Freezing point = 0 - 0.481 = -0.48 C (depression lowers the freezing point, so the sign is negative).

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