The freezing point of pure benzene was measured as 5.49 °C.With the same benzene and same equipment,the freezing point of a solution containing 6.582 g of an unknown molecular compound (sold in the store as "moth crystals")in 100.203 grams of benzene was found to be 3.48 °C.The cryoscopic constant,K_f,for benzene is 5.12 °C kg/mole.Calculate the molar mass weight of the unknown.

Question

Quizplus · Accepted Answer

The freezing point depression (ΔTf) is 5.49 °C - 3.48 °C = 2.01 °C. Using the formula ΔTf = Kf * m, where m is the molality, we find m = 2.01 °C / 5.12 °C kg/mol = 0.3926 mol/kg. The molality is moles of solute per kg of solvent, so moles of solute = 0.3926 mol/kg * 0.100203 kg = 0.0393 mol. The molar mass is mass of solute / moles of solute = 6.582 g / 0.0393 mol = 167 g/mol.

The Freezing Point of Pure Benzene Was Measured as 5