1. Chapter 13-2 Colligative Properties of Solutions
St. Augustine Preparatory School
March 13, 2016

2. 1. Vapor Pressure Lowering
Non-volatile substances have little to no tendency to become a gas under existing conditions.
A non-volatile solute will lower the vapor pressure of the solution having two noticeable effects:
Raising the boiling point of the solution
Lowering the freezing point of the solution
In both cases, more energy is needed than normal to undergo a phase change.

3. 2. Freezing Point Depression
A solution with a nonelectrolyte solute will have a lower freezing point than the pure solvent.
When we dissolve any nonelectrolyte (molecular compound) in water, the freezing point of water will drop ~1.86°C/m.
Each solvent has its own molal-freezing constant (Kf).
Formula: Δtf = Kfm

4. Freezing Point Depression
So how is freezing point depression defined?
The difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent, and its directly proportional to the molal concentration of the solution.
If we double the molal concentration, the freezing-point depression will be doubled.

5. Reminder
Molality: moles of solute divided by kilograms of solvent

6. Example
What is the freezing-point depression of water in a solution of 17.1 g of sucrose (C12H22O11), in 200.g of water? What is the actual freezing point of the solution?

7. Example
A solution contains 117g of sucrose (C12H22O11) in 1300.g of acetic acid (CH3COOHaq). Acetic acid has a normal freezing point of 16.6°C and a freezing point constant of -3.90°C/m. What is the freezing point depression of the solution? What is the new freezing point of the solution?

8. 3. Boiling Point Elevation
A solution with a nonelectrolyte solute will have a higher boiling point than the pure solvent.
A liquid will boil when the vapor pressure of the liquid is equal to the atmospheric pressure.
When we add a nonvolatile substance (one that has little tendency to become a gas under the current conditions), we decrease the vapor pressure of the solution.
This means more energy is needed to boil the solution.

9. Boiling Point Elevation
The boiling-point elevation of a 1-molal solution of any nonelectrolyte solute in water has been found by experiment to be 0.51°C.
Thus, the molal boiling point constant for water is 0.51°C/m.
Formula: Δtb = Kbm

10. Example
What is the boiling point elevation of a solution made from 20.1g of a nonelectrolyte solute and 400.0g of water. The molar mass of the solute is 62.0 g/mol.

11. Practice Problems You will need info from Fig 2.3 on pg 424
A solution contains 45.g of sucrose, C12H22O11, a nonelectrolyte, dissolved in 450.g of camphor. What is the boiling point elevation of the solution? What is the new boiling point? What state is this solution at room temperature?
In a laboratory experiment, the freezing point of an aqueous (dissolved in water) solution of glucose is found to be –0.325°C. What is the molal concentration of the solution?