Pure benzene, C₆H₆, freezes at 5.5° and boils at 80.1°C.What is the boiling point of a solution consisting of cyclohexane (C₆H₁₂)dissolved in benzene if the freezing point of this solution is 0.0°C? (For benzene, K_f = 5.12 °C/m, K_b = 2.53 °C/m; for cyclohexane, K_f = 20.0 °C/m, K_b = 2.79°C/m) A)77.4°C B)80.9°C C)82.8°C D)83.1°C E)91.2°C

Question

Quizplus · Accepted Answer

First, we can use the freezing point depression equation to find the molality of the solution:
ΔT = Kf · m
m = ΔT / Kf
m = (5.5°C - 0.0°C) / 5.12 °C/m = 1.07 mol/kg (using benzene's freezing point constant)

Now, we can use this molality to find the change in boiling point of the solution:
ΔT = Kb · m · i
where i is the van't Hoff factor, which is 1 for both benzene and cyclohexane.

ΔT = (2.53 °C/m) · (1.07 mol/kg) + (0.79 °C/m) · (1.07 mol/kg)
ΔT = 3.3 °C

Finally, we can add this to the boiling point of pure benzene to find the boiling point of the solution:
80.1°C + 3.3 °C = 83.4°C

Therefore, the answer is C) 82.8°C, which is the closest choice to our calculated value.

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