AP Chemistry Lecture 48

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Colligative Properties
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Property whose magnitude depends solely on
the concentration of particles, NOT on the
nature of the particles
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Electrolyte

• solute that forms ions in solution which conducts
  a current

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Non-Electrolyte

• solute that remains as molecules in solution
  which does NOT conduct a current

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molality moles of substance measured through
mass before solution process takes place
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Colligative Properties of Non-Electrolytes
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1. Vapor Pressure Lowering VP Lowering
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1. Vapor Pressure Lowering A solvent B
solute cA cB 1
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1. Vapor Pressure Lowering A solvent B
solute cA cB 1 cA 1 - cB
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but PA cAPoA thus PA
(1 - cB) PoA or PA PoA - cBPoA
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NOTE the lowering of the vapor pressure of the
pure solvent A is a function only of cB
(solute)
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boiling point temperature where vapor pressure
of solvent equals the atmospheric pressure
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2. Boiling Point Elevation
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?Tbp ? m
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?Tbp ? m ?Tbp kbpm where kbp a constant
that is a function of the solvent
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units on kbp ?
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units on kbp ?
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Freezing Water Freezing Solution 3. Freezing
Point Depression
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?Tfp ? m
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?Tfp ? m ?Tfp ? kfp m kfp freezing point
constant
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kfp kbp for the same solvent
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OSMOSIS the flow of solvent into a solution
through a semi-permeable membrane
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Examples Egg Rxn Microscopic Perspective Egg
Explanation Macroscopic Perspective
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4. Osmotic Pressure the pressure that must be
applied to a solution to stop osmosis
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Reverse Osmosis
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4. Osmotic Pressure ? osmotic
pressure ???????? V nB R T
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or ????????? MB R T MB Molarity of solute
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Colligative Properties of Electrolytes
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Must adjust for degree of dissociation
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• i vant Hoff factor
• ?Tbp i kbp m
• ?Tfp i kfp m
• i ??

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Compound i NaCl MgSO4 MgCl2 FeCl3
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Compound i NaCl 1.9 MgSO4
1.3 MgCl2 2.7 FeCl3
3.4
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