please pick up a copy of raoult’s law and vapor pressure problem set distillation problem set
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Gas-Solution ProcessesColligative Properties
Raoult’s Law
Edward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
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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Temp (°C) P (torr)10 9.220 17.530 31.840 55.350 92.5
Vapor PressureWater
On the graph where is the boiling point located?
What does a vapor pressure of water graph look like?
Vapor Pressure of Water
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40
60
80
100
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Temperature (°C)
Pre
ssur
e (t
orr)
What will be the vapor pressure whenthe temperature is 100 °C?
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Vapor Pressure of Water
Which of thedozen water moleculesshown are evaporating?
What factorsdetermine 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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40.0
80.0
120.0
160.0
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Temperature (°C)
Pre
ssur
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orr)
WaterIsopropanol
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Vapor Pressure of a Solution
What factorsdetermine the totalvapor pressure of the solution?
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Raoult’s Law
The vapor pressure of an ideal solution is equal to the sum of the vapor pressure of each component times its mole fraction.
AP BP totalP A B
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P°A is thevapor pressureof pure A.
AP BP totalP A B
If the solventor solute is non-volatile,
then P ° for that species is zero.
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Non-ideal solutions have vapor pressures higher or lower than predicted.
AP BP totalP A B
Raoult’s Law
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Raoult’s 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 isopropanol = 0.878, water = 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 soluteresult in a lower solution vapor pressure
than the pure solvent?
Describe the cause based on a mathematical argumentand a mechanical argument.
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Under what conditions does boiling occur?
Temperature (oC)
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Vapor Pressure ofWater Solutions
Pre
ssu
re (
torr
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WaterSugar Water
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Temperature (oC)
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800
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Vapor Pressure ofWater Solutions
Pre
ssu
re (
torr
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WaterSugar Water
What is the vapor pressure of a 0.10 mole fraction sugar in water solution at 100 °C?
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Temperature (oC)
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200
400
600
800
0 20 40 60 80 100 120
Vapor Pressure ofWater Solutions
Pre
ssu
re (
torr
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WaterSugar Water
If the vapor pressure of a solution is lowered, why is the boiling point elevated?
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Boiling Point Elevation
T 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
T k m ib
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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?
T k m ib
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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.
T k m ib
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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
T k m ib
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Freezing Point Depression
The dissolution of a solute in a solvent results in a lower freezing point for the solution.
T k m if
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Freezing Point Depression
How does an impurity changethe freezing point of the triangular molecules?
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Freezing Point Depression
T is the decrease in freezing point kf is the freezing point (cryoscopic) constant
· kf = 1.86 Km-1 for water Applications
T k m if
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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.
m R T
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Osmotic Pressure
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Osmotic Pressure
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Osmotic Pressure
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Osmotic Pressure
m R T
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Osmotic PressureApplications
m R TMolecularweightdetermination
Biochemical
Why is physiological 0.9% saline solutionimportant 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.