1. Colligative Properties
Vapor Pressure Lowering
Boiling Point Elevation
Freezing Point Depression
Osmosis and Osmotic Pressure

2. What are Colligative Properties?
Solution properties which depend on the concentration of the solute
The identity of the solute is not really important (except…)

3. Vapor Pressure Lowering
A nonvolatile solute is added to a solvent.
The vapor pressure of the solvent lowers!
The more solute is added, the lower the solvent vapor pressure.

4. Vapor Pressure Lowering

5. Vapor Pressure Lowering
This is Raoult’s Law:
P = XsolvP0
P0 is the vapor pressure of pure solvent
P is the vapor pressure of the solvent above the solution

6. Vapor Pressure Lowering
What if the solute is also volatile?
Then both Dalton’s Law of Partial Pressures and Raoult’s Laws are used.

7. Vapor Pressure Lowering
What about ionic compounds?
How many particles does NaCl form when it dissolves?
How many does Ca3(PO4)2 form?
This means that the Xsolv is lower, so the vapor pressure lowers even more!

8. Vapor Pressure Lowering
This is adjusted with a dissociation factor, i
i is the van’t Hoff factor
It is:

9. Phase Diagram What else happens when the vapor pressure lowers?
You can see the whole picture on a phase diagram.

10. Phase Diagram

11. Phase Diagram For a solution, the vapor pressure curve is lowered.
This means that the boiling point of a solution is higher than the boiling point of the pure solvent!

12. Phase Diagram However, the sublimation curve remains unchanged.
After all, this is pure solid to a gas!

13. Phase Diagram The Triple Point thus shifts to a lower temperature.
The Melting Point curve also shifts to a lower temperature.
So the solution freezes at a lower temperature!

14. Boiling Point Elevation
The boiling point for a solution is greater than that of the pure solvent.
You rely on this fact when you put antifreeze in your car in the summer!

15. Boiling Point Elevation
Some people think that this is why we add salt to water, but it’s not!
The equation for this is:

16. Boiling Point Elevation

17. Freezing Point Depression
The freezing point of a sln is lower than that of the pure solvent.
You rely on this when you use antifreeze and when you put salt on an icy sidewalk.

18. Freezing Point Depression

19. Osmotic Pressure, Π Osmotic Pressure is the last colligative property.
It is defined as the pressure necessary to stop a solvent crossing a semipermeable membrane.

20. Osmotic Pressure, Π
Osmotic Pressure uses molarity instead of molality:
Π = MRT

21. Osmotic Pressure

23. Molar Mass Determinations
The molar mass of a compound may be determined through the colligative properties.
By determining the ΔT, we can calculate the molality, the number of moles, and then the molar mass.