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Chapter Outline
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11.1 Energy Changes when Substances Dissolve
11.2 Vapor Pressure
11.3 Mixtures of Volatile Substances
11.4 Colligative Properties of Solutions
11.5 Osmosis and Osmotic Pressure
11.6 Using Colligative Properties to Determine Molar
Mass
Colligative Properties of Solutions
 Colligative Properties:
• Solution properties that depend on concentration
of solute particles, not the identity of particles.
Previous example:
vapor pressure
lowering.
Consequences:
change in b.p.
and f.p. of solution.
© 2012 by W. W. Norton & Company
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Boiling-Point Elevation
and Freezing-Point Depression
 Boiling Point Elevation (ΔTb):
• ΔTb = Kb∙m
• Kb = boiling point elevation
constant of solvent; m = molality.
 Freezing Point Depression (ΔTf):
• ΔTf = Kf∙m
• Kf = freezing-point depression
constant; m = molality.
© 2012 by W. W. Norton & Company
Sample Exercise 11.6: Calculating the Boiling
Point Elevation of an Aqueous Solution
What is the boiling point of seawater? Assume that the
average concentration of sea salt is equivalent to 0.560 m
(molal) NaCl, and that the average density of seawater is
1.025 g/mL.
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Sample Exercise 11.7: Calculating the
Freezing Point Elevation of a Solution
What is the freezing point of an automobile radiator fluid
prepared by mixing equal volumes of ethylene glycol
(MW=62.07) and water at a temperature where the density of
ethylene glycol is 1.114 g/mL and the density of water is 1.000
g/mL? The freezing point depression constant of water, kf, is
1.86 oC/m.
The van’t Hoff Factor
 Solutions of Electrolytes:
• Need to correct for number of
particles formed when ionic
substance dissolves.
 van’t Hoff Factor (i):
•
number of ions in formula unit.
•
e.g., NaCl, i = 2
 ΔTb = i∙Kb∙m & ΔTf = i∙Kf∙m
 Deviations from theoretical
value due to ion pair formation.
© 2012 by W. W. Norton & Company
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Values of van’t Hoff Factors
© 2012 by W. W. Norton & Company
Sample Exercise 11.9:
Using the van’t Hoff Factor
Some Thanksgiving dinner chefs “brine” their turkeys prior to
roasting them to help the birds retain moisture and to season
the meat. Brining involves completely immersing a turkey fro
about 6 hours in a brine (salt solution) prepared by dissolving
2.0 pounds of salt (NaCl, MW=58.44) in 2.0 gallons of water.
Suppose a turkey soaking in such a solution is left for 6 hours
on an unheated porch on a day when the air temperature is
18 oF (-8 oC). Are the brine (and turkey) in danger of
freezing? The kf of water is 1.86 oC/m; assume that i=1.85 for
this NaCl solution.
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Chapter Outline






11.1 Energy Changes when Substances Dissolve
11.2 Vapor Pressure
11.3 Mixtures of Volatile Substances
11.4 Colligative Properties of Solutions
11.5 Osmosis and Osmotic Pressure
11.6 Using Colligative Properties to Determine
Molar Mass
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Molar Mass from Colligative Properties
ΔTf = Kfm and ΔTb = Kbm
Step 1:
solve for molality
ΔTf or ΔTb
m = ΔTf/Kf
kg of solvent
Step 2:
solve for moles
Step 3:
calc. MW
molality
moles
MW = g sample/moles
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Sample Exercise 11.12: Using Freezing
Point Depression to Determine Molar Mass
Eicosene is a hydrocarbon used as an additive in the highpressure fracturing of rock layers to enhance recovery of
petroleum and natural gas, a process sometimes called
“fracking”. The freezing point of a solution prepared by
dissolving 100 mg of eicosene in 1.00 g of benzene is 1.75 oC
lower that the freezing point of pure benzene. What is the molar
mass of eicosene? (kf for benzene is 4.90 oC/m)
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