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An aqueous urea solution (molar mass 56 g mol⁻¹) has a boiling point of 100.18°C at 1 atm pressure. If the values of K_f and K_b for water are 1.86 and 0.512 K kg mol⁻¹, respectively, then the freezing point of this solution is

1. 0.654°C
2. -0.654°C
3. 6.54°C
4. -6.54°C

Correct answer: -0.654°C

Solution

The freezing point depression can be calculated using the formula Δ T_f = K_f · m, where m is the molality of the solution. Given the boiling point elevation and the properties of water, the calculated freezing point depression results in a freezing point of -0.654°C, indicating that the solution freezes at a lower temperature than pure water.

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