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Kf for water is 1.86 K kg mol−1. If your automobile radiator holds 1.0 kg of water, how many grams of ethylene glycol (C2H6O2) must you add to get the freezing point of the solution lowered to −2.8 °C?

1. 72 g
2. 93 g
3. 39 g
4. 27 g

Correct answer: 93 g

Solution

molality = dTf/Kf = 2.8/1.86 = 1.505 mol/kg. For 1.0 kg water this is 1.505 mol of ethylene glycol. Mass = 1.505 * 62 = ~93 g.

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