Pure cyclohexane, C₆H₁₂, has a freezing point of 6.53°C. Its freezing point depression constant is: K_f = 20.0 °C m^-1. A solution was made by taking 18.55 g of an unknown nonelectrolyte and dissolving it in 150.0 g of cyclohexane. The measured freezing point of the solution was -4.28°C. Calculate the molecular weight of the unknown substance.Hint: Find molality, then moles, then molar mass, being sure to keep track of units.

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Quizplus · Accepted Answer

The freezing point depression is calculated as ΔTf = 6.53°C - (-4.28°C) = 10.81°C. Using the formula ΔTf = Kf * m, where m is the molality, we find m = 10.81°C / 20.0°C m^-1 = 0.5405 m. Molality is moles of solute per kg of solvent, so moles of solute = 0.5405 m * 0.150 kg = 0.081075 moles. The molar mass is then 18.55 g / 0.081075 moles = 229 g/mol.

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