Thyroxine, an important hormone that controls the rate of...

Thyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland. When $0.455 \mathrm{~g}$ thyroxine is dissolved in $10.0 \mathrm{~g}$ benzene, the freezing point of the solution is depressed by $0.300^{\circ} \mathrm{C}$. What is the molar mass of thyroxine? See Table $11.5$.

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Key Concepts

Colligative Properties

Colligative properties are physical properties of solutions that depend solely on the number of solute particles present and not on their identity. These properties include freezing point depression, boiling point elevation, osmotic pressure, and vapor pressure lowering. They provide a useful means to determine molar masses of solutes by analyzing how the addition of a non-volatile solute alters the physical properties of the solvent.

Freezing Point Depression

Freezing point depression is the phenomenon where the freezing point of a solvent decreases when a solute is added. This lowering of the freezing point is directly proportional to the concentration of the solute particles in the solution. By measuring the magnitude of the depression and knowing the cryoscopic constant of the solvent, one can calculate the molality of the solution, which is crucial for further determining the molar mass of the solute.

Molality

Molality is defined as the number of moles of solute per kilogram of solvent and is a concentration unit that is independent of temperature. It is particularly useful in colligative property calculations, such as freezing point depression, because it directly relates the amount of solute to the effect observed on the solvent without being affected by volume changes due to temperature.

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