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

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

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.

Vapor Pressure Lowering

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

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

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!

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

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

Phase Diagram

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!

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

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!

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!

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

Boiling Point Elevation

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.

Freezing Point Depression

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.

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

Osmotic Pressure

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.