Chem 105 Tuesday 8 Feb 2011
Chapter 14: Properties of Solutions
Solutions defined: dilute conc saturated etc
Aqueous solubility and Ionic compounds
Review molarity - clicker
Solutions: Concentration measurements
Molality (m)
Mass %
Mole Fraction (X)
Dissolving Gases and Henry’s Law
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Do you agree?
1. Yes
2. No
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No
Ye
s
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2
Solutions: Solute (sugar or salt)
Solvent (water)
Dilute -> concentrated -> saturated (-> supersaturated).
If both solid and solution together in
flask, the maximum amount of solid
has dissolved.
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NaCH3CO2
Most ionic compounds are more
water soluble at higher
temperatures.
(Difficult to predict Temp behavior
because all the forces in the solid,
solvent, and solution are
temperature dependent.)
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Supersaturated sodium
acetate (NaCH3CO2)
Made by dissolving ~1:2
water and solid sodium
acetate at 100 °C, and
cooling to room
temperature (carefully!).
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(The sodium acetate
will stay in solution if
there are no dust
particles to act as
nucleation sites.)
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Solubility of Ionic Compounds is determined by balance between forces:

+
Hsolution
enthalpy
S
entropy
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+- attractive
forces in crystal
ordered
+ many H-bonds
ordered

ion-dipole IMF
(fewer H-bonds;
no crystal forces)
disordered
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Ca2+(g) + 2 Cl- (g)
ΔHhydration Ca2+  Ca2+ (aq)
H
ΔHlattice
2 ΔHhydration Cl-  2 Cl- (aq)
CaCl2 (s)
Ca2+ (aq) + 2 Cl- (aq)
ΔHSolution =  ΔHhydration - ΔHlattice
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ΔHSolution < 0 Soluble; ΔHSolution > 0 Insoluble
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Molarity, molality, and all that
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First Let’s Review:
Moles Solute
Molarity (M) 
Liter of Solution
mol L
We prepare solutions of defined molarity (M) to facilitate
practical laboratory procedures.
That is, so we can dispense “moles of chemicals by volume”.
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Using a 1.00-liter volumetric flask, a student made up a solution
of NaCl with a final concentration of 0.100 M. Exactly how much
water did she add to the flask?
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1.100 liter
1.000 liter
0.900 liter
Not determined from
information given.
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22
d.
..
r
m
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lit
e
N
ot
0.
de
te
r
90
0
lit
e
0
00
1.
1.
10
0
lit
e
r
21
r
1.
2.
3.
4.
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ANSW: 4. Not determined
All we know is that “enough
water” was added to make
up the volume to the 1.00-L
line on neck of the
volumetric flask.
(It would be less than 1.00 L)
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IN CONTRAST,
the properties of solutions depend on the exact ratio of
solvent to solute.
So, DIFFERENT concentration functions are defined that
precisely define the amounts of solute and solvent.
Molality ( m) 
Mass % 
moles solute
kg solvent
mass solute
x100
mass solute  mass solvent
Mole Fraction ( X ) 
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moles solute
moles solute  moles solvent
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20.6
mass% 
Hint: Always choose a definite amount
mass NH 3
x100
mass NH 3  mass H 2O
Choose a definite amount, say 100.0 g. If the solution
is 26.0 % ammonia, then 26.0 g is ammonia, and the rest
74.0 g is water.
M olality and other concentrat ion functions are
" intensive" properties. That is, they apply toany
amount. So, apply it to this 100.0 - g sample :

Hint: Sometimes the density is given only
as a decoy and should be ignored.
(However, density IS needed in molarityto-molality calculations.)
1 mol NH 3
moles NH 3
17.03 gNH 3
m

kg solvent 74.0 g H O  1 kg H 2O
2
1000 g H 2O
26.0 gNH 3 
m  20.631 m  20.6 m
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Do another concentration problem on OWL…
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Dissolving Gases & Henry’s Law
Solubility of gas in liquid = Constant x Pressure of the gas
Sg = kHPg
where kH is called the Henry’s Law Constant and is different for each gas
and temperature.
Sgas (mol/kg) = kH(mol/kg-bar) Pgas (bar)
Henry’s Law different units of concentration and gas pressure may be used
X, M, m, ppm atm, mmHg, bar
Sgas (mol/L) = kH(mol/L-atm) Pgas (atm)
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As you release the pressure on a bottle of carbonated
water or soda, the CO2 gas comes out of solution.
CO2 dissolved in water. There are
hydrogen-bonds and dipole-induced
dipole interactions around the
molecule. (Recall that CO2 is non-polar
due to its linear symmetric shape,
therefore its permanent dipole = 0.0.)
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A CO2 bubble is ~1% the
density of water - it is
mostly empty space.
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CO2
O2
S
N2
1
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P ->
www.wikipedia.org17
Which do you predict has a larger Henry’s Law
constant?...
1. methane
2. ethane
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e
ha
n
et
m
et
ha
ne
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Which do you predict has a larger Henry’s Law
constant?...
1. methane
2. ethane
CH4
C2H6
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Ethane is a larger molecule, with a
greater surface area than
methane. Thus it is more
polarizable and should be able to
interact with solvent more strongly
by dipole- induced dipole forces.
15.0 x 10-4
20.0 x 10-4
http://www.mpch-mainz.mpg.de/~sander/res/henry.html
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Henry’s Law problem from OWL. nope
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Application of Henry’s Law
(1) Civil and environmental engineers use Henry’s Law to calculate
vapor pressure of volatile organic compounds given a known
concentration in contaminated ground water.
Pgas = Sgas/kH
These are complex calculations, because
1. kH is temperature dependent. (Gases are less soluble at higher
temperatures.)
2. kH may depend on other substances dissolved in aqueous phase.
Example groundwater contamination site in Illinois
http://www.epa.state.il.us/community-relations/fact-sheets/gem-cleaners/gem-cleaners-2.html
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Hemoglobin protein (red showing backbone only)
binds heme molecule (gray)
and Fe atom (purple),
which carries O2 (white).
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90% of the red blood cell is
hemoglobin.
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(2) Solubility of O2 in blood
O2 is not very soluble in water: at 37°C it is about 0.01 mM,
which is not enough to sustain metabolism in a large
organism.
Blood contains about 2.2 mM* hemoglobin in red blood
cells. Each hemoglobin carries a maximum of 4 O2 molecules,
so the max [O2] in oxygenated blood is about 8.8 mM.
This almost 1000x higher than the natural solubility of O2.
*1 mM = 10-3 M
To see the hemoglobin-O2 complex, visit http://chem.uaf.edu/keller/Molecules/Myoglobin.html
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