Colligative Properties
What are Colligative Properties?
•
•
Properties which depend on the concentration of the solute
The identity of the solute is not really important
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.
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
• What about ionic compounds?
• How many particles does NaCl form when it dissolves?
• How many does Ca (PO ) form?
• This means that the X is lower, so the vapor pressure lowers even more!
3
4 2
solv
•
•
•
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.
• 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!
• However, the sublimation curve remains unchanged.
• After all, this is pure solid to a gas!
• 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!
• Some people think that this is why we add salt to water, but it’s not!
• The equation for this is:
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.
Osmotic Pressure, • Osmotic Pressure is the last colligative property.
• It is defined as the pressure necessary to stop a solvent crossing a semipermeable
membrane.
• Osmotic Pressure uses molarity instead of molality:
= MRT
Molar Mass Determinations
• The molar mass of a compound may be determined through the colligative
properties.
• By determining the T, we can calculate the number of moles, and then the molar
mass.