Download Solute Concentration: Molality

5/25/2012
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.
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Solute Concentration: Molality
 Changes in boiling point/freezing point of
solutions depends on molality:
m
moles of solute
kg of solvent
• Preferred concentration unit for properties
involving temperature changes because it
is independent of temperature.
© 2012 by W. W. Norton & Company
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Calculating Molality
Starting with:
a) Mass of solute
and solvent.
b) Mass of solute/
volume of solvent.
c) Volume of solution.
© 2012 by W. W. Norton & Company
Sample Exercise 11.8
How many grams of Na2SO4 should be added to
275 mL of water to prepare a 0.750 m solution of
Na2SO4? Assume the density of water is 1.000
g/mL.
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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 11.9
What is the boiling point of seawater if the
concentration of ions in seawater is 1.15 m?
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Sample Exercise 11.10
What is the freezing point of radiator fluid prepared
by mixing 1.00 L of ethylene glycol (HOCH2CH2OH,
density 1.114 g/mL) with 1.00 L of water (density
1.000 g/mL)? The freezing-point-depression
constant of water, Kf, is 1.86°C/m.
© 2012 by W. W. Norton & Company
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.
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Values of van’t Hoff Factors
© 2012 by W. W. Norton & Company
Sample Exercise 11.11
The salt lithium perchlorate (LiClO4) is one of the most
water-soluble salts known. At what temperature does a
0.130 m solution of LiClO4 freeze? The Kf of water is
1.86°C/m; assume i = 2 for LiClO4 and the freezing point
of pure water is 0.00°C.
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Osmosis
 Osmosis:
• Movement of solvent through semi-permeable
membrane from region of low solute
concentration to region of higher solute
concentration.
 Osmotic Pressure (π):
• Pressure required to halt flow of solvent
through membrane due to osmosis.
• π = iMRT (M = molarity of solution)
© 2012 by W. W. Norton & Company
Osmosis at the Molecular Level
Direction of solvent flow.
Osmotic Pressure (π)
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Osmosis: Medical Application
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Reverse Osmosis
 Use of high pressure to move solvent across
membrane from region of high solute concentration
to region of lower solute concentration.
 Application: desalination/water purification.
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Molar Mass from Colligative Properties
Calculations:
Sample Exercise 11.13
At the beginning of this section, we mentioned that the
concentration of solutes in a red blood cell is about a
third of that of seawater—more precisely, about 0.30 M.
If red blood cells are bathed in pure water, they swell, as
shown in Figure 11.22(c). Calculate the osmotic
pressure at 25°C of red blood cells across the cell
membrane from pure water.
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Sample Exercise 11.15
What is the reverse osmotic pressure required at 20°C to
purify brackish well water containing 0.355 M dissolved
particles if the product water is to contain no more than
87 mg of dissolved solids (as NaCl) per liter?
© 2012 by W. W. Norton & Company
Sample Exercise 11.16
Eicosene is a molecular compound and nonelectrolyte
with the empirical formula CH2. The freezing point of a
solution prepared by dissolving 100 mg of eicosene in
1.00 g of benzene was 1.75°C lower than the freezing
point of pure benzene. What is the molar mass of
eicosene? (Kf for benzene is 4.90°C/m.)
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Sample Exercise 11.17
A molecular compound that is a nonelectrolyte was
isolated from a South African tree. A 47 mg sample was
dissolved in water to make 2.50 mL of solution at 25°C,
and the osmotic pressure of the solution was 0.489 atm.
Calculate the molar mass of the compound.
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