Colligative Properties
Vapor Pressure Lowering,
Freezing Point Depression,
Boiling Point Elevation,
and Osmotic Pressure
Copyright 2003 - John Sayles
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Colligative Properties
Def’n: Properties in which solutions differ
from the pure solvent.
Depend on the number of dissolved
particles, independent of their identity.
i, the van t’hoff factor, plays a role, though
Great way to count moles for solutes
Copyright 2003 - John Sayles
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Colligative Properties
We will assume all solutes are non-volatile
Electrolytes are solutions containing ionic
solutes
Ionic solutes break into 2 or more ions and so
cause 2 or more times the colligative effect
Account for this with the van t’Hoff factor, I
Non-electrolytes contain covalent solutes
i = 1
Copyright 2003 - John Sayles
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Vapor Pressure Lowering
The VP of a sol’n is less than the VP of the
pure solvent
Why: the solute molecules block the
surface of the sol’n making it tougher for
the solvent to vaporize
Depends on mole fraction
If half of the molecules at the surface are
solute, then the solvent is half as likely to
vaporize. VP is thus lowered by 50%.
Copyright 2003 - John Sayles
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Demonstration of Vapor-Pressure Lowering
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©
12-15
Houghton Mifflin Company. All rights reserved
Copyright 2003 - John Sayles
5
VPL Calculation (Raoult’s Law)
VPsol’n = VPpure solvent • Xsolvent
VPL = VPpure solvent • Xsolute
or
or
Logical approach: In a 30% sol’n, 30% of
the surface sites are blocked by solute, so
30% of the VP is lost.
Applications: causes BP elevation, salt
water less volatile than fresh water
Copyright 2003 - John Sayles
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VPL Problems
The VP of pure water at room temp is 20.
mmHg. What is the VP of a 20% sugar
sol’n? What is the VP of a 20% NaCl sol’n?
With a volatile solute: What is the vapor
pressure of a 70% toluene/30% benzene
solution at room temp? The VP’s of pure
toluene and benzene are 22 mmHg and 75
mmHg at room temp, respectively.
Copyright 2003 - John Sayles
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A Better Problem (on board)
20.0 g of an unknown solid is dissolved in
125 g of water at 25oC. What is the
molecular weight of the solid if the
observed vapor pressure was 21.72 torr
and pure water usually has a vapor
pressure of 23.76 torr at this temperature?
Answer: 30.87 g/mol
Copyright 2003 - John Sayles
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Non-ideal solutions
Raoult’s Law predicts a linear relationship
between VP an X
For a non-ideal sol’n, the relationship is
not perfectly linear
An ideal solution is one where the
attractions between solute and solvent are
similar to the solute-solute and solventsolvent attractions
Copyright 2003 - John Sayles
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Boiling Point Elevation
A direct result of VPL (recall def’n of BP)
Applications: antifreeze, hot cocoa
Calculation: ∆Tb = kb • m • i
kb is the molal BP elevation constant
kb is a function of the solvent (page 523)
Copyright 2003 - John Sayles
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Using BPE to find Moles
∆Tb = kb • m • i
Dissolve X grams of solute in Y kg solvent
Measure the ∆Tb
Look up kb for the solvent, figure out i for the
solute
Calculate m = ∆Tb/(kb • i)
m • kg solvent = moles solute
(and usually MW = g X/moles X)
Do Alternate Example 12.12
Copyright 2003 - John Sayles
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FP Depression
The solvent is harder to freeze because it has
to squeeze out the solute as it freezes
Homemade popsicles, thawed fudgesicles
Applications: antifreeze, salt on icy roads,
salt in ice cream making, ice cubes in diet
pop, diet pop freezing
Calculation: ∆Tf = kf • m • i (just like BPE)
Do ASA for FP Depression Lab
Copyright 2003 - John Sayles
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Osmotic Pressure
Only occurs in the presence of a
semipermeable membrane
Many biological membranes are SPM’s
With SPM, diffusion of solute is replaced
by osmosis of water
As water osmoses into the membrane, the
pressure inside the membrane enclosure
increases
Copyright 2003 - John Sayles
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Semipermeable Membrane
12_21
Water
molecule
G lu
cos
e
Gluco
se
G lu
cos
Glu
cos
e
e
Membrane
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©
12-21
Houghton Mifflin Company. All rights reserved
Copyright 2003 - John Sayles
14
An Experiment in Osmosis
Copyright
©
12-22
Houghton Mifflin Company. All rights reserved
Copyright 2003 - John Sayles
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Osmotic Pressure
∏ V = nRT
or ∏ = M•RT
Applications: blood pressure, cell
lysis/dehydration, salt curing, isotonic
medical fluids, produce turgidity, RO
purification
Copyright 2003 - John Sayles
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