Please Pick Up a Copy of

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Please Pick Up a Copy of

• Raoults Law and Vapor Pressure problem set
• Distillation problem set

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Gas-Solution ProcessesColligative
PropertiesRaoults Law

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Colligative Properties

• Reading assignment
• Zumdahl Chapter 16.10-16.11, 17.5-17.7
• The properties of a solution depends on the
  amount of dissolved solute in the solution.
  Properties which depend on the amount of solute,
  but not the identity of the compound are called
  colligative properties.

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Vapor PressureWater
What does a vapor pressure of water graph look
like?
On the graph where is the boiling point located?
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Vapor Pressure of Water
What will be the vapor pressure when the
temperature is 100 C?
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Vapor Pressure of Water
Which of the dozen water molecules shown are
evaporating?
What factors determine whether a molecule will
evaporate?
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Which Is More Volatile?Water or Isopropanol
Vapor Pressure of Water Isopropanol
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Vapor Pressure of a Solution
What factors determine the total vapor pressure
of the solution?
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Raoults Law

• The vapor pressure of an ideal solution is equal
  to the sum of the vapor pressure of each
  component times its mole fraction.

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If the solvent or solute is non-volatile, then P
for that species is zero.
PA is the vapor pressure of pure A.
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Raoults Law

• Non-ideal solutions have vapor pressures higher
  or lower than predicted.

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Raoults Law Application

• Distillation The separation of one component
  from a solution by boiling and condensation of
  the vapor.

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Benzene-Toluene Distillation

• One mole of benzene is mixed with three moles of
  toluene.
• What will be the vapor pressure of the solution
  at 88 C.
• Benzene (C6H6) and toluene (C6H5CH3) form an
  ideal solution.
• Pbenzene is 960 torr at 88 C
• Ptoluene is 380 torr at 88 C

What other questions could be asked?
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Benzene-Toluene Distillation

• One mole of benzene is mixed with three moles of
  toluene.
• What will be the composition of the vapor that is
  condensed?

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Distillation

• Distillation separates one compound from another
  because the vapor and the solution have different
  compositions.
• How could you make a solution containing two
  volatile components very pure?

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Fractional Distillation
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Not All Compounds can be Separated by Distillation

• Non-ideal solutions that have the same
  composition in solution and the vapor are called
  azeotropes.
• Isopropanol (BP 82 C) and water (BP 100 C)
  forms a 80.4 C azeotrope of composition
  Cisopropanol 0.878, Cwater  0.122

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Colligative Properties

• A physical property of a solution which depends
  upon the concentration of the solute, but not its
  nature or identity.
• Examples
• Vapor pressure lowering
• Boiling point elevation

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Vapor Pressure Lowering
Why does the dissolution of a non-volatile
solute result in a lower solution vapor
pressure than the pure solvent?
Describe the cause based on a mathematical
argument and a mechanical argument.
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Under what conditions does boiling occur?
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What is the vapor pressure of a 0.10 mole
fraction sugar in water solution at 100 C?
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If the vapor pressure of a solution is lowered,
why is the boiling point elevated?
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Boiling Point Elevation

• DT is the increase in boiling point
• kb is the boiling point (ebullioscopic) constant
• kb 0.512 Km-1 for water
• m is the molality of the solution

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Boiling Point Elevation

• Sugar water does not conduct electricity.
• kb 0.512 Km-1 for water
• What is the boiling point of 0.10 mole fraction
  sugar water solution?

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Boiling Point Elevation

• i is the multiplicative factor to account for
  ions
• if a compound does not ionize then there is one
  particle per molecule and i 1.
• if a compound breaks into two ions then i
  2, etc.

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Rank the following solutions in terms of
increasing boiling point

• water
• 0.2 m salt water
• 0.3 m sugar water

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Freezing Point Depression

• The dissolution of a solute in a solvent results
  in a lower freezing point for the solution.

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Freezing Point Depression
How does an impurity change the freezing point of
the triangular molecules?
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Freezing Point Depression

• DT is the decrease in freezing point
• kf is the freezing point (cryoscopic) constant
• kf 1.86 Km-1 for water
• Applications

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Osmotic Pressure

• A pressure differential due to solvent molecules
  passing through a semipermeable membrane from a
  region of lower solute concentration to a region
  of higher solute concentration.

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Osmotic Pressure
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Osmotic Pressure
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Osmotic Pressure
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Osmotic Pressure
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Osmotic PressureApplications
Molecular weight determination
Biochemical
Why is physiological 0.9 saline
solution important in intravenous transfusions?
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Historical Perspective

• Colligative properties were important to the
  early chemists because it provided information
  about the molecular weight or degree of
  ionization of the solute.