Arginine vasopressin is a pituitary hormone. It helps...

Arginine vasopressin is a pituitary hormone. It helps regulate the amount of water in the blood by reducing the flow of urine from the kidneys. An aqueous solution containing $21.6 \mathrm{mg}$ of vasopressin in $100.0 \mathrm{~mL}$ of solution has an osmotic pressure at $25^{\circ} \mathrm{C}$ of $3.70 \mathrm{mmHg}$. What is the molecular mass of the hormone?

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Key Concepts

Osmotic Pressure

Osmotic pressure is a colligative property that measures the pressure required to stop the net flow of solvent molecules across a semipermeable membrane separating a solution from pure solvent. It is directly proportional to the concentration of dissolved particles in the solution and can be used to determine molecular masses by linking the pressure with the molarity, temperature, and gas constant through the equation ? = MRT.

Colligative Properties

Colligative properties are characteristics of solutions that depend solely on the number of solute particles rather than their identity. These properties include osmotic pressure, boiling point elevation, freezing point depression, and vapor pressure lowering. They are particularly useful in determining molar masses of large molecules, as the effect is proportional to the particle number in a solution.

Molecular Mass Determination Using Colligative Properties

This concept involves calculating the molar mass of a solute by measuring a colligative property, such as osmotic pressure, in a given solution. By rearranging the equation ? = MRT to solve for the number of moles and using the known mass of the solute, the molar mass can be determined. This method is especially important in biochemistry for determining the molar masses of proteins and hormones.

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