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Colligative Properties


Colligative Properties Chapter 13, Section 2 * What is a colligative property? A property that depends on the concentration* of solute particles NOT the identity ... – PowerPoint PPT presentation

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Title: Colligative Properties

Colligative Properties
  • Chapter 13, Section 2

What is a colligative property?
  • A property that depends on the concentration of
    solute particles NOT the identity of the
  • Vapor Pressure Reduction
  • Boiling Point Elevation
  • Freezing Point Depression
  • Osmotic Pressure
  • Concentration will be in terms of molality (m)

  • Vapor pressure is pressure caused by molecules in
    the gas phase that are in equilibrium with the
    liquid phase. (in a closed system particles go
    back and forth between phases)

Vapor Pressure Reduction
  • The pressure of the vapor over a solvent is
    reduced when a solute is dissolved in it. Vapor
    pressure reduction is directly related to
    concentration of the solution.

  • Why does this occur?
  • Increasing solute particles reduces the
    proportion of solute to solvent. Fewer solvent
    particles will be available to leave the solution
    and enter the gas phase.
  • The solution will remain a liquid over a larger
    temp. range.

Phase Diagram
Shows us what phase the solvent will be in at a
given temperature and pressure.
If we add a solute that lowers the vapor
pressure, then our phase diagram adjusts
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Boiling Point Elevation
  • The boiling point of a solvent is raised when a
    solute is dissolved in it. It is directly
    related to the concentration of the solution.
  • Boiling point the temp. at which vapor pressure
    of a liquid is equal to atmospheric pressure

  • The change in boiling point is proportional to
    the molality of the solution
  • ?Tb Kb ? m
  • Kb is the molal boiling point elevation
    constant, a property of the solvent. It has the
    units ºC/m.
  • ?Tb is the boiling point elevation. It is the
    difference between the boiling points of the pure
    solvent and that of the solution.

Freezing Point Depression
  • The freezing point of a solvent is lowered when a
    solute is dissolved in it. Freezing point
    depression is directly related to concentration
    of a solution.
  • Freezing point the temp. where vapor pressure
    of the solid and liquid phases are the same.

  • The change in freezing point can be found
  • ?Tf Kf ? m
  • Here Kf is the molal freezing point depression
    constant of the solvent.

Osmotic Pressure (?)
  • The pressure required to prevent osmosis.
  • Osmosis is the flow of particles from a dilute
    solution to a concentrated one across a semi
    permeable membrane.

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Electrolytes change moles of solute
  • Because ionic compounds dissociate in water, they
    will increase the concentration of dissolved
  • When determining or using moles of an
    electrolyte, use the dissociated amount.
  • AlBr3 (s) ? Al3 (aq) 3 Br- (aq)

  • If 50.0 g of BaCl2 are added to 2.5 kg of
    water, compute the change in freezing point and
    state the new freezing point of the solution.
  • Water Kf -1.86?C/m Kb.51 ?C/m

Drill 4/29/11
  • Write everything you know about acids and bases

  • Determine the freezing point of a solution of
    60.0g of glucose, C6H12O6, dissolved in 80.0g of

  • Workbook pg. 226 1, pg. 229 1, pg. 232 1
  • (use the table on pg. 223 for Kb or Kf)
  • Then pg. 233 3, 11 and pg. 234 17
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