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Gas-Solution ProcessesGas SolubilityRaoult’s Law
Edward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
04/18/23
Solutions
Reading assignment: Fine, Beall & Stuehr: Chapter 9.1, 9.3
A homogeneous mixture of two or more substances in a single phase.• Solute
· The material dissolved in a solution.• Solvent
· The medium into which a solute is dissolved to form a solution.
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Solutions
What is the solute and what is the solvent?• An ounce of isopropanol (rubbing alcohol)
is added to a cup of water.• A drop of water is added to a cup of
isopropanol.• A cup of water is added to a cup of
isopropanol.
MolalityOne cup of isopropanol is mixed with one cup of water.
isopropanol
CH3-CHOH-CH3
0.78 gmL-1
water
1.00 gmL-1
Molality (m) = moles of solute per kilogram of solvent
What is the molality of the solution?
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Henry’s Law
Assumes thatthe gas and the solventdon’t react with each other.
mdissolved gas = k · Pgas
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Henry’s Law
mdissolved gas = k · Pgas
dissolved gas = k’ · Pgas
The concentration of the gas in solutioncould also be defined in terms ofmole fraction of gas in solution.
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Henry’s Law
Partial Pressure of Gas (torr)0
0mo
les
of
dis
solv
ed g
asp
er k
ilo
gra
m o
f s
olv
ent
(m)
slope = Henry’s Law constant (k)
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Solubility of Gases in Water1 atm gas pressure above solution
CH4
O2
CO
N2
He
Temperature (ºC)
Sol
ubili
ty
(mm
ol/k
g so
lven
t)
00
1
2
10 20 30
What effect does heating have onthe amount of dissolved gas?
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Solubility of Gases
The solubility of gas in liquids usually decreases with increasing temperature.
However, some gases become more soluble in organic solvents with increasing temperature.
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Carbonated Beverages
mdissolved CO2 = 0.034 m·atm-1 · PCO2
Packaged under about2 atm of CO2 pressure.
Why does acarbonated beverage
go "flat" overnight?
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Diver's Bends
Caused by dissolved nitrogen in the blood.• A diver can go deep for a short period of
time, if nitrogen gas does not have a chance to equilibrate in the blood.
• How do the partial pressures of nitrogen and oxygen in the blood compare at 1 atm pressure and under 33 feet of water?
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Physiological ImplicationsHenry’s Law
What are the problems associated with breathing pure oxygen for a sustained period?• How do scuba divers avoid breathing high
partial pressures of oxygen? Why is it harder to work and breathe in the
mountains?• Why do athletes train in the mountains?
Gas-Solution ProcessesColligative Properties
Edward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
04/18/23
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
0
20
40
60
80
100
0 10 20 30 40 50
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
0
40
80
120
160
0 10 20 30 40 50
Temperature (°C)
Pre
ssur
e (t
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?
Microsoft Excel Worksheet
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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 beSeparated 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)
0
200
400
600
800
0 20 40 60 80 100 120
Vapor Pressure ofWater Solutions
Pre
ssu
re (
torr
)
WaterSugar Water
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Temperature (oC)
0
200
400
600
800
0 20 40 60 80 100 120
Vapor Pressure ofWater Solutions
Pre
ssu
re (
torr
)
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)
0
200
400
600
800
0 20 40 60 80 100 120
Vapor Pressure ofWater Solutions
Pre
ssu
re (
torr
)
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.
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