Thyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland. If 0.455 g of thyroxine is dissolved in 10.0 g of benzene, the freezing point of the solution is 5.144°C. Pure benzene freezes at 5.444°C and has a value for the molal freezing-point-depression constant of K_f of 5.12°C/m. What is the molar mass of thyroxine? A) 2330 g/mol B) 285 g/mol C) 777 g/mol D) 3760 g/mol E) 777,000 g/mol

Question

Quizplus · Accepted Answer

The freezing point depression is calculated using ΔTf = Kf * m, where ΔTf = 5.444°C - 5.144°C = 0.300°C. The molality (m) is then 0.300°C / 5.12°C/m = 0.0586 mol/kg. Using the mass of the solute (0.455 g) and the solvent (10.0 g = 0.010 kg), the molar mass (M) is calculated as M = 0.455 g / (0.0586 mol/kg * 0.010 kg) = 777 g/mol.

Thyroxine, an Important Hormone That Controls the Rate of Metabolism