Colligative Properties

Colligative properties

Properties that depend only on the number of solute particles in solution, not their identity.

Four colligative properties

1. Vapor pressure lowering 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure

Vapor pressure lowering

The vapor pressure of a solution is lower than that of the pure solvent due to fewer solvent molecules at the surface.

Raoult's Law

Psolution = Xsolvent ⋅ Psolventᵒ, where X is the mole fraction and Pᵒ is vapor pressure of pure solvent.

Boiling point elevation

The boiling point of a solution is higher than that of the pure solvent.

Formula for boiling point elevation

ΔTb = i ⋅ Kb ⋅ m, where ΔTb is the increase in boiling point, i is the van’t Hoff factor, Kb is the ebullioscopic constant, and m is molality.

Freezing point depression

The freezing point of a solution is lower than that of the pure solvent.

Formula for freezing point depression

ΔTf = i ⋅ Kf ⋅ m, where ΔTf is the decrease in freezing point, i is the van’t Hoff factor, Kf is the cryoscopic constant, and m is molality.

Osmotic pressure

The pressure needed to stop the flow of solvent through a semipermeable membrane into a more concentrated solution.

Formula for osmotic pressure

Π = i ⋅ M ⋅ R ⋅ T, where Π is osmotic pressure, i is the van’t Hoff factor, M is molarity, R is the gas constant, and T is temperature in Kelvin.

Van’t Hoff factor (i)

The number of particles the solute breaks into in solution (e.g., NaCl → 2, CaCl₂ → 3).

Effect on osmotic pressure with more solute

Osmotic pressure increases, because more solute increases concentration.

Effect on boiling point with more solute

Boiling point increases due to boiling point elevation.

Effect on vapor pressure with more solute

Vapor pressure decreases since solute particles block evaporation.

True or False: Adding solute lowers the boiling point of a solution.

False. It raises the boiling point.

True or False: Adding solute lowers the vapor pressure of a solution.

True. Vapor pressure goes down as solute is added.

True or False: Adding solute increases osmotic pressure.

True. Higher solute concentration equals higher osmotic pressure.