Colligative-Properties

Colligative Properties of

Solutions

  • Definition: Colligative properties depend solely on the number of particles in solution, not the type of particles.

  • Types of Colligative Properties:

    • Vapor pressure lowering

    • Freezing point depression

    • Boiling point elevation

    • Osmotic pressure

Lowering of Vapor Pressure and Raoult’s Law

  • Nonvolatile solute lowers vapor pressure due to fewer solvent molecules at the surface.

  • Raoult’s Law:

    • Formula: P_solution = X_solvent * P_pure solvent

    • Valid for ideal solutions where ΔH_solution = 0.

Boiling Point Elevation

  • Nonvolatile solute raises boiling point above that of the pure solvent.

  • Due to lowered vapor pressure needing higher temperature to match atmospheric pressure.

Freezing Point Depression

  • Nonvolatile solute lowers freezing point relative to the pure solvent.

  • Formula: ΔT_f = K_f * m

    • K_f = freezing point depression constant.

Osmotic Pressure

  • Definition: Osmosis is solvent flow from lower to higher concentration solutions through a semipermeable membrane.

  • Osmotic pressure is affected by solute concentration and can be calculated using π = MRT.

Colligative Properties and Dissociation of Electrolytes

  • Electrolytes have greater effects than nonelectrolytes due to higher particle release.

  • Van’t Hoff Factor (i): Accounts for number of ions produced in solution; affects calculations of boiling point elevation and freezing point depression.

    • i = number of ions from dissociation.

Key Formulas

  1. Raoult’s Law: P_solution = X_solvent * P_pure solvent

  2. Boiling Point Elevation: ΔT_b = K_b * m

  3. Freezing Point Depression: ΔT_f = K_f * m

  4. Osmotic Pressure: π = MRT