Powerpoint for Colligative Properties
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Transcript Powerpoint for Colligative Properties
Colligative Properties
Colligative properties
Properties that depend on the TOTAL number
of dissolved particles
Colligative properties
Properties that depend on the TOTAL number
of dissolved particles
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure
Vapor pressure lowering
Adding solute leads to
more intermolecular
attractions
It becomes harder for
solvent molecules to
escape into the gas
phase
Vapor pressure lowering
More solute lower
vapor pressure than
pure solvent
Raoult’s Law
PA X P
o
A A
Effects of non-volatile solutes
Boiling Point Elevation
Tbp m kb
Freezing Point Depression
Tfp m k f
Electrolytes vs. nonelectrolytes
Colligative properties depend on total number
of dissolved particles
Electrolytes vs. nonelectrolytes
Colligative properties depend on total number
of dissolved particles
Non-electrolytes don’t dissociate
Electrolytes DO dissociate into ions in solution
Van’t Hoff factor, i
For non-electrolyte, i = 1
For electolytes, i = # of ions in formula
(theoretical maximum)
Predict the van’t Hoff factor:
Glucose, C6H12O6
NaCl
AlCl3
Methanol, CH3OH
Ion pairing
One mole of NaCl does
not yield two moles of
ions
Some ions will
reassociate for a short
time
The actual van’t Hoff
factor will be slightly
lower than predicted
van’t hoff factors
More ion pairing occurs at higher
concentrations
van’t hoff factors
More ion pairing occurs at higher
concentrations
Incorporating the van’t Hoff
factor
Boiling point elevation:
Tbp m i kb
Incorporating the van’t Hoff
factor
Boiling point elevation:
Tbp m i kb
Freezing point
depression:
Tfp m i k f
Problem
33.5 g of potassium chloride are dissolved in 459 g of water.
Calculate the boiling point and freezing point of the resulting
solution.
Problem
17.8 g of an unknown solute are dissolved in 276 g of water. If the
new freezing point is -1.67oC, calculate the molar mass of the
unknown substance.