Freezing Point Depression

Introduction to Freezing Point Depression

When a solute is added to a pure solvent, the freezing point of the solvent decreases. This phenomenon is known as freezing point depression, a type of colligative property that depends on the number of solute particles in a solution, not their identity.

• Normal Freezing Point: The temperature at which a pure solvent solidifies under standard atmospheric pressure.

• Freezing Point of Solution: The temperature at which a solution solidifies, which is lower than that of the pure solvent.

Key Terms and Definitions

• Colligative Properties: Properties that depend on the concentration of solute particles, not their chemical identity (e.g., boiling point elevation, freezing point depression, osmotic pressure).

• Freezing Point Depression (ΔTf): The decrease in the freezing point of a solvent upon addition of a solute.

• Molality (m): The number of moles of solute per kilogram of solvent.

• Cryoscopic Constant (Kf): A proportionality constant specific to each solvent, representing the freezing point depression per molal concentration of a non-volatile solute.

• Van't Hoff Factor (i): The number of particles into which a solute dissociates in solution. For non-electrolytes, i = 1.

Freezing Point Depression Formula

The freezing point depression can be calculated using the following equation:

• = change in freezing point (°C)

• = van't Hoff factor (number of particles per formula unit)

• = freezing point depression constant (°C·kg/mol)

• = molality (mol solute/kg solvent)

Constants for Water

Example Calculation

Example: Calculate the freezing point of a solution containing 110.7 g glucose (C6H12O6) dissolved in 302.6 g water.

• Moles of glucose:

• Mass of water: 302.6 g = 0.3026 kg

• Molality:

• For glucose, (non-electrolyte)

• Freezing point of solution:

Practice Problems

• Problem 1: How many moles of ethylene glycol (C2H6O2) must be added to 1,000 g of water to form a solution that has a freezing point of -10°C?

• Problem 2: An ethylene glycol solution contains 28.3 g of ethylene glycol in 97.2 mL of water. Calculate the freezing point of the solution. (Density of water = 1.00 g/mL)

• Problem 3: When 825 g of an unknown is dissolved in 3.45 L of water, the freezing point of the solution is decreased by 2.89°C. Assuming the unknown compound is a non-electrolyte, calculate its molar mass.

Summary Table: Freezing Point Depression Variables

Applications

• Antifreeze in car radiators uses ethylene glycol to lower the freezing point of water, preventing engine damage in cold climates.

• Salting roads in winter lowers the freezing point of water, helping to melt ice.