Colligative Properties Learning Target: What are the four colligative properties of solutions? Determine the boiling point elevation and freezing point depression of a solution.

Main Idea Colligative properties depend on the number of solute particles in a solution.

Colligative Property Colligative properties are physical properties of solutions that are affected by the number of particles but not by the identity of dissolved solute particles. Ionic compounds are electrolytes because they dissociate in water to form a solution that conducts electricity. Some molecular compounds are also electrolytes.

4 colligative properties: Vapor Pressure Reduction Boiling Point Elevation Freezing point depression Osmotic Pressure

Vapor Pressure Reduction Molecules of nonvolite solute take up space, thus preventing some solvent molecules to vaporize. Condensation continues at the same rate, but vaporization slows down. Since condensation > vaporization, vapor pressure is reduced.

Boiling Point Elevation (ΔTb) Adding a nonvolite solute, reduces vapor pressure, more energy is needed to make the solutions boil, raising the boiling point. Boiling point elevation = BP of the solution - BP of the solvent. Directly proportional to the number of solute molecules.

Boiling Point Elevation

Boiling Point Elevation (ΔTb) ΔTb = Kbm Kb = boiling point elevation constant m= molality

Freezing Point Depression (ΔTf) The ability of a solute to lower freezing point of the solution. Directly proportional to the molality of the solute. ΔTf = Kfm Kf = freezing point depression constant m= molality

B.P. & F.P. – colligative property Simulation

Osmotic Pressure Osmosis: The movement of water molecules from high water concentration to low water concentration. Osmotic pressure is the amount of additional pressure caused by water molecules that moved into the concentrated solution.

Osmotic Pressure & types of solutions Isotonic concentration: solute concentration is the same on both sides of the membrane. Hypertonic: higher solute concentration than the solution Hypotonic: lower solute concentration than the solution

Examples 1. How much will the boiling point of water be elevated if 100. g of sucrose (C12H22O11) is added to 500.g of water? Kb for water is 0.52 °C/m. ΔTb = kbx m

Example 2 What is the freezing point depression when 153g of bromine is added to 1000. g of benzene?