1. Colligative Properties

2. Colligative: particles are particles
Colligative properties depend on the amount of solute but not on the identity of the solute
The effect is the same for all solutes
The magnitude of the effect is solely dependent on the amount or concentration of the solute

3. Colligative Properties for Nonvolatile Solutes in Liquid
Osmotic pressure drives the solvent from less concentrated to higher concentrated solutions
Vapor pressure - always lower
Boiling point - always higher
Freezing point - always lower
Why does this occur?

4. Osmosis: Molecular Discrimination
A semi-permeable membrane discriminates on the basis of molecular type (size or charge)
Solvent molecules pass through
Large molecules or ions are blocked
Solvent molecules will pass from a place of lower solute concentration to higher concentration to achieve equilibrium

5. Osmotic pressure Solvent passes into more concentrated solution
The movement of the solvent can be prevented by application of pressure
The pressure to prevent solvent movement is the osmotic pressure

6. Calculating osmotic pressure
We borrow from the ideal gas law
Since n/V = M…
Where M is the molar concentration of particles and Π is the osmotic pressure
R is the Gas Law Constant
What is i?

7. What is i? The symbol i is the van’t Hoff factor.
It is dependent on dissociation.
It is the numbers of particles produced per mole of substance in solution
1 mole of molecular solid → 1 mole of particles
1 mole of NaCl → 2 moles of particles
1 mole of CaCl2 → 3 moles of particles 7

8. Vapor Pressure
The pressure of the vapor resulting from evaporation of a liquid above a sample of the liquid (or solid) in a closed container

9. Non-volatile solutes and Raoult’s law
Vapor pressure of solvent in solution containing non-volatile solute is always lower than vapor pressure of pure solvent at same temperature
Solute particles occupy volume reducing rate of evaporation of the solvent molecules at the surface
The rate of evaporation decreases and so the vapor pressure above the solution must decrease

10. Raoult’s Law
Vapor pressure above solution is vapor pressure of solvent times mole fraction of solvent in solution
In an ideal setting, the nonvolatile solute would not vaporize, and therefore, would not contribute to the vapor pressure.

11. A mixture of two volatile liquids
Total pressure in a mixture of two volatile liquids i.e. benzene and toluene equals the sum of vapor pressures of each components 11

12. Implications on Boiling Point
Remember when the vapor pressure equals the atmospheric pressure, boiling occurs.
T0 represents the boiling point of pure solvent and T1 represents the boiling point of solution.

13. Magnitude of BP Elevation
The change in BP (ΔTb) can be calculated by:
Kb is the molal boiling point elevation constant – it is a constant for each solvent.
For water, the Kb is K kg mol-1.

14. What is molality? Molality (m) is the ratio of solute to solvent
Molality = mole of solute/kg of solvent
Molarity (M) is the ratio of solute to soultion
Molarity = mole of solute/Liter of solution

15. Freezing and Boiling

16. Freezing Point Depression

17. Magnitude of FP Depression
The change in FP (ΔTf) can be calculated by:
Kf is the molal freezing point depression constant – it is a constant for each solvent.
For water, the Kf is 1.86 K kg mol-1. 17

18. Deviation from the Ideal Solution
Like ideal gas law, Raoult’s Law works for an ideal solution
Real solutions behave most like ideal solutions
At lower concentrations
Solute – solvent sizes are similar
Solute –solvent attraction is minimal
Solvent – solvent interactions are stronger than the solute – solvent: Pvap is higher
Solvent – solute interactions are stronger than solvent – solvent interactions: Pvap is lower

19. Learning objectives Describe meaning of colligative property
Calculate osmotic pressure in solution and use to determine molar mass of solute
Use Raoult’s law to determine vapor pressure of solutions
Describe physical basis for vapor pressure lowering
Predict magnitude of boiling point elevation
Predict magnitude of freezing point depression
Determine the van’t Hoff factor of a substance
Explain why solution deviate from ideal solutions