Chapter 13- Unit 2 Colligative Properties - are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties/identity (e.g. size or mass) of the molecules
Vapor-Pressure Lowering Nonvolatile Substance- A substance that has little tendency to become a gas under existing conditions. Adding a nonvolatile solute to a solvent always lowers the vapor pressure.
Freezing-Point Depression Freezing-Point Depression(Δtf)- the difference between the freezing points of the pure solvent and a solution of a non- electrolyte in that solvent, and it is directly proportional to the molal concentration of the solution Δtf = Kfm Molal freezing point constant (Kf)- the freezing point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute. Molality (m) – mol solute/kg of solvent
Freezing-Point Depression Con’t
Practice Problem A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of -0.23°C. What is the molal concentration of the solution? m= / Kf Δtf = f.p of solution – f.p of pure solvent = -0.23°C – 0.00°C = -0.23°C m= 0.12m Δtf
Boiling-Point Elevation Boiling-Point Elevation- the difference between the boiling points of the pure solvent and a solution of a non-electrolyte in that solvent, and it is directly proportional to the molal concentration of the solution Δtb= Kbm *Molal freezing point constant (Kb) Example - antifreeze
Osmotic Pressure Osmosis- The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration Semipermeable membrane- allows the passage of some particles while blocking the passage of others Osmotic Pressure- the external pressure that must be applied to stop osmosis Because osmotic pressure is dependent on the concentration of solute particles and not on the type of solute particles, it is a colligative property The greater the concentration of a solution, the greater the osmotic pressure