Chapter 13 part 4: Colligative
Properties
Read:
HW:
BLB 13.5-13.6
BLB 13:9, 58, 61, 67, 69, 75
Packet 13:13 - 18
How many particles in Solution?
Know: Colligative Properties, Colloids
!vapor pressure lowering
Raoult’s Law: PA = XAP° A
!boiling point elevation
"Tb = Kbm
!freezing point depression
"Tf = Kfm
!osmotic pressure
! n$
# = # & RT = MRT
" V%
!"#$%&'%("#%$#)(%*+,%-#./0&$#11
____________________________________
When/Where is Exam 3?? _____________
When is the LATE DROP DEADLINE?
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Dr. L. S. Van Der Sluys
Page 1
COLLIGATIVE PROPERTIES ARE:
Properties that depend on the
Colligative Properties
0.2M solution of Pb(NO3)2
Is it an electrolyte?
What is the dissolution process?
Concentration Pb+2 ?
Concentration NO3! ?
total concentration of particles =
0.5M solution of CH3COOH (acetic acid)
Is it an electrolyte?
What is the dissolution process?
Solution has a concentration of
particles greater than _____ but less
than _______
Examples of Colligative properties
!vapor pressure lowering
!boiling point elevation
!freezing point depression
!osmotic pressure
Dr. L. S. Van Der Sluys
Page 2
Colligative Properties
VAPOR PRESSURE LOWERING:
Addition of a nonvolatile solute to a solvent
lowers the vapor pressure
Phase Diagrams for a pure solvent and
for a solution of a nonvolatile solute
How is melting point affected by the
solute particles?
pure solvent
solvent + solute
(non-volatile)
How does this affect the:
rate of evaporation?
vapor pressure of SOLVENT?
Raoult’s Law: PA = XAP° A
PA $ vapor pressure of solution
XA $ mole fraction of solvent
P° A $ 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.
Dr. L. S. Van Der Sluys
Page 3
Colligative Properties
How is boiling point affected by the solute
particles?
Dr. L. S. Van Der Sluys
Page 4
Colligative Properties
BOILING POINT ELEVATION
FREEZING POINT DEPRESSION
Increase in boiling point:
"Tb = Kbm
where Kb is molal boiling point elevation constant
Decrease in freezing point:
"Tf = Kfm
where Kf is molal freezing point depression constant
m $ molality of the solution
water
benzene
1. 0.2M NH4NO3
2. 0.2M Pb(NO3)2
3. 0.2M Na2SO4
4. 0.2M AlPO4
Kb and Kf are tabulated for different solvents
Eg.
Which of the following solutions will have the
lowest boiling point? (Choose all correct
answers)
Kb= 0.512°C/m
Kf= 1.86°C/m
Kb= 2.53°C/m
Kf= 5.12°C/m
5. 0.2M AlBr3
6. 0.5M CH3COOH (acetic acid)
Example:
Ocean salinity is about 35 g salt per 1 kg seawater.
What is the freezing point of seawater assuming all
of the salinity* is due to NaCl? (MW of NaCl = 58.5
g/mol)
Which salt would you use to melt ice on the
sidewalk, if you wanted to use the smallest
quantity to do the job?
1.
2.
3.
4.
5.
MgSO4 (Epsom salts)
NaCl
CaCl2
KI
they would all work the same
2
*seawater contains mainly: Cl–, Na+, SO4 ! , Mg2+,
Ca2+, and K+
Dr. L. S. Van Der Sluys
Page 5
Colligative Properties
Dr. L. S. Van Der Sluys
Page 6
Colligative Properties
Molecular Weight Determination
Osmosis
Flow of molecules through a membrane
Stepwise problem solving
1. From "Tb or "Tf calculate m for solution
(molality).
2. From m and knowledge of how solution was
prepared, calculate MW.
Example:
A solution of 12 g of glucose in 100 g H2O boils at
100.340°C. What is the MW of glucose?
"Tb = Kbm
m=
Dr. L. S. Van Der Sluys
Page 7
Colligative Properties
Dr. L. S. Van Der Sluys
Page 8
Colligative Properties
Mixtures
Osmotic pressure (#) is:
Particle size
Small
n
# = !# $& RT = MRT
" V%
units of atm
Medium
Large
name
molarity
Reverse Osmosis: (Used to purify water)
1. Exert pressure on a concentrated
solution, forcing solvent molecules
through a membrane.
2. Solute molecules are trapped in the
concentrated portion.
uniformity
particles
examples
Osmosis through red blood cell walls
Colloids
1.
2.
3.
Dr. L. S. Van Der Sluys
Page 9
Colligative Properties
Dr. L. S. Van Der Sluys
Page 10
Colligative Properties
Tyndall effect: light is scattered when
% of light & particle size
Why is the sky blue?
Light Scattering
setting
sun
sky
Earth
For molecules (~5Å) % in X-ray region
" visible light passes through solutions without
scattering
" Atmosphere scatters small % light
" Explains why the sky is blue.
mid-day sun
For colloids (size~ 200nm) % is in visible region:
• Light passes through colloidal suspensions
with scattering (milk, fog).
sky
Earth
Colloids scatter blue light more than
red light.
Dr. L. S. Van Der Sluys
Page 11
Colligative Properties
Dr. L. S. Van Der Sluys
Page 12
Colligative Properties
Hydrophilic vs. hydrophobic colloids
• hydrophilic: water-loving
• hydrophobic: water-fearing
• (water-soluble) proteins: hydrophobic
core with hydrophilic surface
• detergents:
hydrophobic tail
with hydrophilic
head
Dr. L. S. Van Der Sluys
Page 13
Colligative Properties