Colligative Properties of Solutions Summary

Colligative Properties of Solutions

• Depend on concentration of solute; independent of solute identity.

Key Colligative Properties

1. Osmotic Pressure: Pressure required to prevent osmosis.
2. Vapor Pressure Lowering: Addition of solute lowers vapor pressure of solvent.
3. Boiling Point Elevation: Higher solute concentration increases boiling point.
4. Freezing Point Depression: Addition of solute lowers freezing point.

Concepts

• Osmosis: Solvent particles diffuse through a semipermeable membrane.
• Osmotic Pressure: Exerting pressure prevents water diffusion to a concentrated side.
• Boiling Point Elevation Formula: ΔTb = Kb * m
• Freezing Point Depression Formula: ΔTf = Kf * m

Boiling Point Elevation Examples

• For a 1.5 m sucrose solution: Calculate using ΔTb.
• Sample Problem: 0.37 molal lactic acid in water: New boiling point = 100.19 °C.
• 120 g lactic acid in 600 g cyclohexane: New boiling point = 84.18 °C.

Freezing Point Depression Examples

• 0.25 molal lactic acid: New freezing point = -0.465 °C.
• 120 g lactic acid in cyclohexane: New freezing point = -18.24 °C.

Electrolytes and Colligative Properties

• Colligative properties relate to solute particles.
• Measured values much higher than calculated suggest solute is an electrolyte that dissociates.

Formulas for Electrolytes

• Boiling Point Elevation: AT₁ = iK₁m
• Freezing Point Depression: ATƒ = -iKƒm

Van't Hoff Factor (i)

• Nonelectrolytes: i ≈ 1
• Electrolytes: i varies with concentration.

Common Electrolytes and Van't Hoff Factors

Example Problem

• 82.20 g of CaCl2 in 400 g water:
• Freezing point = -10.34 °C
• Boiling point = 102.85 °C