1. Colligative Properties
Solutions
Colligative Properties

2. Colligative Properties
Properties that depend on the concentration of solute particles but NOT on their identity
Nonvolatile substance:
A compound that has little tendency to become a gas under existing conditions

3. Vapor – Pressure Lowering
The pressure of a vapor in equilibrium with its liquid phase
Can be thought of as a measure of the particles tendency to escape into the vapor phase
How does the vapor pressure of a solvent change when it contains a nonvolatile solute?
It is lowered

4. Freezing – Point Depression
Molal freezing-point constant (Kf):
The freezing-point depression in a 1-molal solution of a nonvolatile, nonelectrolyte solute
Freezing-point depression (∆tf):
The difference between the freezing points of the pure solvent and a solution of nonelectrolyte in that solvent

5. Freezing – Point Depression
molality
Measured in oC/molal
Measured in oC

6. Freezing – Point Depression
Example: A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of oC. What is the molal concentration of the solution?
Kf = oC/m for water

7. Freezing – Point Depression
Example: A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of oC. What is the molal concentration of the solution?

8. Boiling – Point Elevation
Molal boiling-point constant (Kb):
The boiling-point elevation in a 1-molal solution of a nonvolatile, nonelectrolyte solute
Boiling-point elevation (∆tb):
The difference between the boiling points of the pure solvent and a solution of nonelectrolyte in that solvent

9. Boiling – Point Elevation
molality
Measured in oC/molal
Measured in oC

10. Boiling – Point Elevation
Example: What is the boiling-point elevation of a solution made from 20.0 g of a nonelectrolyte solute and g of water? The molar mass of the solute is 62.0 g/mol.
Kf = 0.51 oC/m for water

11. Boiling – Point Elevation
Example: What is the boiling-point elevation of a solution made from 20.0 g of a nonelectrolyte solute and g of water? The molar mass of the solute is 62.0 g/mol.
Step 1: find the molality

12. Boiling – Point Elevation
Example: A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of oC. What is the molal concentration of the solution?