Lecture 30-32

Solutions
Colligative Properties
•  vapor pressure lowering
Raoult s Law: PA = XAP°A
•  boiling point elevation
!Tb = Kbm
•  freezing point depression
!Tf = Kfm
•  osmotic pressure
#n&
" = % ( RT = M RT
$V'
Colloids
!
Mary J. Bojan
Chem 110
1
Test Question
Which of the following aqueous solutions has the
greatest total concentration of ions?
1.  0.2M NH4NO3
2.  0.2M Pb(NO3)2
3.  0.2M Na2SO4
4.  0.2M AlPO4
5.  0.2M AlBr3
6.  0.2M CH3COOH (acetic acid)
Mary J. Bojan
Chem 110
2
COLLIGATIVE PROPERTIES are properties that
depend on the number of solute particles
(collections) in solution but not on the identity
of the solute.
Examples of Colligative properties
•  vapor pressure lowering
•  boiling point elevation
•  freezing point depression
•  osmotic pressure
Mary J. Bojan
Chem 110
3
COLLIGATIVE PROPERTIES
Example: What is the total particle concentration in a
0.2M solution of Pb(NO3)2 ? (salt, strong electrolyte)
[Pb+2]=
[NO3"]=
total concentration of particles =
What is the total particle concentration 0.2M solution of
CH3COOH (acetic acid)? (weak acid, weak electrolyte)
CH3COOH(aq) ! H+(aq) + CH3COO"(aq)
Mary J. Bojan
Chem 110
4
When a non volatile solute is dissolved in a
volatile solvent, the VAPOR PRESSURE of
the solvent will be LOWERED
VAPOR PRESSURE LOWERING
rate of evaporation !
vapor pressure of SOLVENT !
Mary J. Bojan
Chem 110
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DEMO: VAPOR PRESSURE LOWERING
Mary J. Bojan
Chem 110
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Quantitatively: for dilute solutions the partial
pressure of the solution PA is related to the
mole fraction and the vapor pressure of the
pure solvent P°A
Raoult s Law: PA = XAP°A
PA =
XA =
P°A =
vapor pressure of solution
mole fraction of solvent
vapor pressure of pure solvent
Vapor pressure lowering is a colligative property
It depends on the concentration but not on the nature of the solute.
Mary J. Bojan
Chem 110
7
Lowering of the vapor pressure results in
changes in the melting and boiling points.
BOILING POINT ELEVATION
FREEZING POINT DEPRESSION
!Tb = Kbm
Kb = molal boiling point elevation constant
!Tf = Kfm
Kf = molal freezing point depression constant
m = molality of the solution
Kb and Kf are tabulated for different solvents
Eg.
water
benzene
Mary J. Bojan
Kb= 0.52°C/m
Kf= 1.86°C/m
Kb= 2.53°C/m
Kf= 5.12°C/m
Chem 110
Don’t forget that
these are
colligative
properties!!!
8
Example
What is the freezing point of seawater assuming all of the
salinity* is due to NaCl?
(MW of NaCl = 58.5 g/mol)
*ocean salinity is ca. (35 g salt)/(1 kg seawater)
mainly: Cl–, Na+, SO42", Mg2+, Ca2+, K+
Mary J. Bojan
Chem 110
9
Osmosis: flow of molecules through a
membrane
Osmotic pressure:
#
Pressure needed to stop the flow of molecules through a
membrane
#n&
" = % (RT = MRT
$ V'
units of atm
molarity
!
Mary J. Bojan
Chem 110
10
Osmosis through a cell membrane
Hypertonic: solution:
water moves out of cell
Hypotonic solution:
water moves into cell;
can cause cell to rupture.
Isotonic: solution that has same osmotic pressure as cell.
Mary J. Bojan
Chem 110
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Reverse Osmosis: exert pressure on a
concentrated solution, forcing solvent
molecules through a membrane.
Solute molecules are trapped in concentrated portion.)
Used to purify water.
Mary J. Bojan
Chem 110
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An aqueous solution contains 3.50 mg of protein dissolved in sufficient
water to form 5.00 mL of solution. The osmotic pressure of the solution
at 25°C was found to be 1.54 torr.
Calculate the molar mass of the protein.
Mary J. Bojan
Chem 110
13
The size of particles in a mixture influences
the properties of the mixture.
Particle Size
small
solution
uniform
molecules
Colloidal dispersion
uniform
Particle size = 20-2000 Å
large
mixture
non-uniform
Sedimentary
particles
NaCl(aq)
Sugar(aq)
EXAMPLES
Milk (fat particles)
Fog (water droplets)
silt
Mary J. Bojan
Chem 110
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Colloid: homogeneous mixture of particles
defined by the size of the particles
• 
• 
• 
• 
Gravity does not cause particles to settle
between solution and heterogeneous mixture
particles are very large molecules or aggregates
phases are mutually insoluble
Mary J. Bojan
Chem 110
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How light interacts with matter: Light scattering
Tyndall effect: light is scattered when $ of light % particle size
For molecules (~5Å) $ in X-ray region
•  visible light passes through solutions without scattering
•  Atmosphere scatters small $ light
Explains why the
sky is blue
For colloids (size~ 200nm) $ is in visible region:
Light passes through colloidal suspensions with scattering (milk, fog).
Mary J. Bojan
Chem 110
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Beautiful colors found in stained glass windows
are made using Gold Colloids
•  Elixir of Life, a potion made
from gold (ancient Rome)
•  Used to stain glass
Solutions of gold nanoparticles:
colors are related to size of the gold
particles suspended in solution
Mary J. Bojan
Chem 110
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