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

wThyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland. When 0.455 g of thyroxine is dissolved in $10.0 \mathrm{g}$ of benzene, the freezing point of the solution is depressed by $0.300^{\circ} \mathrm{C} .$ What is the molar mass of thyroxine? See Table 17.5.

Key Concepts

Colligative Properties

These are properties of solutions that depend only on the number of solute particles present in a given amount of solvent, not on the identity of the solute. The effect on the freezing or boiling point, as seen in this problem, is a typical example of a colligative property.

Freezing Point Depression

This is one of the colligative properties where the freezing point of a solvent is lowered by the addition of a nonvolatile solute. The depression in the freezing point is directly proportional to the molal concentration of the solute in the solution.

Cryoscopic Constant

Also known as the freezing point depression constant, this is a proportionality constant specific to each solvent, used in the formula ?T = Kf · m. It quantifies how much the freezing point of the solvent is lowered per molal concentration of the solute.

Molality

Molality is a concentration unit defined as the number of moles of solute per kilogram of solvent. It is used in colligative property calculations because it does not change with temperature, unlike volume-based concentration units.

Molar Mass Determination

Determining the molar mass of a compound using colligative properties involves measuring the extent of freezing point depression, calculating the molality of the solution, and then relating the known mass of the solute to its amount in moles to find its molar mass.

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