When a 18.4-g sample of an unknown compound is dissolved in 522.0 g of benzene, the freezing point of the resulting solution of 3.82°C. The freezing point of pure benzene is 5.48°C, and K_f for benzene is 5.12°C/m. Calculate the molar mass of the unknown compound. A) 9.20 * 10³ g/mol B) 4.72 *10¹ g/mol C) 8.85 * 10² g/mol D) 2.96 * 10¹ g/mol E) 1.09 * 10² g/mol

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Quizplus · Accepted Answer

The change in freezing point (ΔTf) is 5.48°C - 3.82°C = 1.66°C. Using the formula ΔTf = Kf * m, where m is the molality, we find m = 1.66°C / 5.12°C/m = 0.3242 mol/kg. The molality is moles of solute per kg of solvent, so moles of solute = 0.3242 mol/kg * 0.522 kg = 0.1692 mol. The molar mass is mass of solute / moles of solute = 18.4 g / 0.1692 mol = 108.7 g/mol, which is approximately 1.09 x 10^2 g/mol.

When a 184-G Sample of an Unknown Compound Is Dissolved in 522