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Chapter 12
Solutions and Osmosis
Dr. Peter Warburton
[email protected]
http://www.chem.mun.ca/zcourses/1011.php
Pure substances
We’ve seen that the physical
properties of a pure
substance are dependent on
the strength of the
intermolecular forces
between the molecules
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Solutions
Solutions are “mixtures” of two
or more pure substances. The
substance found in greater
amounts is usually referred to as
the SOLVENT, while all other
substances in the solution are
considered to be the SOLUTE(S).
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Solutions
Solutes and solvents only tend to
mix (are MISCIBLE) when the
intermolecular forces of the
solvent and solute(s) are of
similar strengths.
“Like dissolves like”
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Figure 12.5
If molecules A and B have
large differences in
intermolecular force
strengths, then a molecule
of A will be “sucked
back” into pure A instead
of mixing with pure B.
Oil (London forces) and
water (hydrogen bonds)
DON’T MIX, and are said
to be IMMISCIBLE.
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Figure 12.6
If molecules A and B
have similar
intermolecular force
strengths, then a
molecule of A can mix
with pure B.
Alcohols (hydrogen
bonds) and water
(hydrogen bonds)
DO MIX!
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Physical properties of solutions
Even though “like dissolves like” the
presence of solute molecules in the
solvent disrupts the bulk solvent
intermolecular forces to some extent.
This change in forces means that the
physical properties of solutions, like
freezing and boiling points, are slightly
different than those of the pure solvent.
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Physical properties of solutions
The disruption of forces generally
depends more on the amount of solute
(in terms of concentration) rather than
the chemical identity of the solute.
Such colligative properties of solutions
therefore depend on the concentration
of the solution while ignoring the identity
of the solute.
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Dilution of solutions
If we take a solution and add
pure solvent to it, we expect
dilution to occur.
The solution and solvent mix
until a new solution of lower
concentration is made.
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Dilution of solutions
If we instead take a solution
and separate it from pure
solvent using a
semipermeable membrane
that solute molecules can’t
pass through, then we are
“controlling the mixing.”
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Osmosis
Osmosis is the process where
the solvent passes through
the semipermeable
membrane to EQUALIZE the
solute concentration on both
sides of the membrane.
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Osmotic pressure
Osmotic pressure is a
colligative property of a solution
that is defined as the external
pressure that must be applied
to a solution to just stop the
process of osmosis through the
membrane.
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Osmotic pressure
Osmotic pressure P depends
on the concentration of the
solution (M), the temperature
(T) and the gas constant (R).
P = MRT
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Osmotic pressure and molar mass
1.
2.
3.
4.
5.
Make a solution of a certain mass of
solute in a given total volume.
Measure the osmotic pressure.
Calculate M from measured P and T.
Use M and volume to calculate
moles of solute.
Use moles and mass of solute to
calculate molar mass.
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Problem 12.44
An aqueous solution of 21.6 mg
of vasopressin in 100.0 mL of
solution has an osmotic
pressure at 25 C of 3.70 mmHg.
What is the molar mass of the
hormone?
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Osmosis and biology
Cell walls are often semipermeable
membranes that allow water to pass
through, but not many solute
molecules.
Body fluids, like blood plasma, must
have concentrations that match those
inside the cell or osmosis might
cause cells to shrink or rupture.
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Blood cells and osmosis
Normal blood cell
in isotonic (same
concentration)
solution.
Collapsed blood cell
in hypertonic (more
concentrated)
solution.
Bloated blood cell in
hypotonic (less
concentrated)
solution.
No osmosis.
Water leaves the
cell during osmosis.
Water enters the
cell during osmosis.
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