When a 19.4-g sample of an unknown compound is dissolved in 458.0 g of benzene, the freezing point of the resulting solution of 3.82°C. The freezing point of pure benzene is 3.86°C, and K_f for benzene is 5.12°C/m. Calculate the molar mass of the unknown compound. A) 7.47 × 10³ g/mol B) 5.62 × 10¹ g/mol C) 9.34 × 10² g/mol D) 2.81 × 10¹ g/mol E) 1.34 × 10² g/mol

Question

Quizplus · Accepted Answer

The change in freezing point (ΔTf) is 0.04°C. Using the formula ΔTf = Kf * m, where m is the molality, we find m = 0.04°C / 5.12°C/m = 0.0078125 mol/kg. The molality is moles of solute per kg of solvent, so moles of solute = 0.0078125 mol/kg * 0.458 kg = 0.003578 mol. The molar mass is mass of solute / moles of solute = 19.4 g / 0.003578 mol = 1.34 × 10^2 g/mol.

When a 194-G Sample of an Unknown Compound Is Dissolved in 458