COLLIGATIVE PROPERTIES
Depend on the number of solute
particles in solution but not on the
identity of the solute
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure
EXAMPLES
0.5 m solution of Pb(NO3)2
0.5 m Pb2+ and 1.0 m NO3– → 1.5 m total ions
0.5 m HC2H3O2 (acetic acid)
HC2H3O2
H+ + C2H3O2–
solution is between 0 and 1.0 m in total ions
solution is between 0.5 and 1.0 m in all species
EXPRESSING
CONCENTRATION
weight
percent
=
mass component
x 100
total mass
mole fraction
moles component
=
of component
total moles
molarity =
moles solute
liters solution
moles solute
molality =
mass solvent (kg)
(%)
(fraction)
(M)
(m)
ELECTROLYTES
A substance that yields ions when
dissolved in water is an electrolyte
Strong electrolytes
completely ionized in solution
good conductors
Weak electrolytes
partially ionized in solution
poor conductors
Nonelectrolytes
not ionized in solution
non-conductors
STRONG & WEAK
ELECTROLYTES
STRONG
NaCl(s) + H2O → Na+(aq) + Cl–(aq) + H2O
salt completely ionized
HCl(aq) + H2O → H3O+(aq) + Cl–(aq)
complete ionization of strong acid
WEAK
NH4+(aq) + OH–(aq)
NH3(aq) + H2O
partial ionization of weak base or weak acid
H3O+(aq) + C2H3O2–(aq)
HC2H3O2(aq) + H2O
NON
C6H12O6(s) + H2O → C6H12O6(aq) + H2O
glucose
no ionization
Flowchart for identifying electrolytes
note: ionic compounds are strong electrolytes
but they could be insoluble(!!!)
• memorize strong acids and bases (BLB
Table 4.2); If a compound is an acid or a base,
but NOT one of the strong acids or bases, then
it MUST be a weak electrolyte
• Common misconception: electrolytes are ionic
compounds: this is NOT TRUE (e.g., HCl)
Sheets
Page 3
Lecture 18
Which of these aqueous solutions has
the greatest total concentration of ions?
Which has the least?
1.
2.
3.
4.
0.4 M NH4NO3
0.2 M Pb(NO3)2
0.3 M Na2SO4
0.2 M AlPO4
5. 0.5 M C6H12O6 (sugar)
VAPOR PRESSURE
LOWERING
Raoult’s Law
vapor pressure
of solution
PA = XA PAo
vapor pressure
pure solvent
mole fraction
of solvent
vapor pressure lowering is a colligative
property — its depends on the conc
but not on the nature of the solute
RAOULT’S LAW EXAMPLE 1
Calculate total vapor pressure of a liquid at room
temperature that is composed of a mixture of
benzene and toluene. The mole fractions of
benzene and toluene are
Xben = 0.33 and Xtol = 0.67.
o
Benzene: Pben
= 75 torr
Toluene:
Ptolo = 22 torr
PA = XA PAo
RAOULT’S LAW EXAMPLE 2
Calculate the vapor pressure at 25 oC of a solution
made by adding 50.00 mL of glycerin (C3H8O3, a
nonvolatile nonelectrolyte with a density of 1.26
g/mL) to 500.0 g of water. The vapor pressure of
pure water at 25 oC is 23.8 torr.
PA = XA PAo
BOILING POINT ELEVATION
FREEZING POINT DEPRESSION
Shift in vapor pressure, and shift
in phase diagram explains
ΔTf and ΔTb
ΔTb = Kb m
molal BP elevation const
ΔTf = Kf m
molal FP depression const
molality of the solution
(solute particles)
FREEZING POINT DEPRESSION
SEAWATER
Ocean salinity ~ 35 g salt / 1 kg seawater
Cl–
SO42– Mg2+
Na+
Ca2+
K+
molar mass of NaCl is 58.5 g/mol
35 g
= 0.60 mol NaCl
58.5 g/mol
x2
1.2 mol
= 1.2 molal
m=
1000 g seawater
Kf = 1.86 °C/m for H2O
ΔTf = Kf m = (1.86)(1.2) = 2.23 °C
Tf = – 2.23 °C
FREEZING POINT DEPRESSION
EXAMPLE
Choose the solute which would decrease the freezing
point to -5 oC of a solution that is made by dissolving
0.0538 moles of the substance in 100 g of water. The
freezing point depression constant of water is 1.86 oC/m
A.
B.
C.
D.
E.
NaF
CaCl2
Al(NO3)3
Ca3(PO4)2
C6H12O6
OSMOTIC PRESSURE
dilute
solution
concentrated
solution
semipermeable
membrane
movement continues until osmotic pressure
builds up to stop it
π = MRT
M is molarity of particles
Example: cucumber in brine loses water
by osmosis to make pickle
Osmosis
• flow of molecules through
a semi-permeable
membrane; NET movement
of
is toward
solution with higher solute
concentration; movement
of solvent continues until
osmotic pressure builds
up to stop it
• osmotic pressure ( ):
pressure needed to
of a molecule
through a membrane
n
= RT = MRT
V • is osmotic pressure (what
units will this be in???)
• R is gas constant in (L atm)/(mol K)
• T is temperature in K
• M is concentration in molarity (mol/L)
• what does this equation remind you of???
Sheets
Page 10
Lecture 20
Consequence of osmotic pressure
• red blood cells: the cell membrane of red
blood cells is a semi-permeable membrane
cell in hypertonic soln
cell in hypotonic soln
Sheets
Page 12
Lecture 20
OSMOTIC PRESSURE
EXAMPLE
Honey is 82% sugar by mass. Most of the rest is
water. Bacteria love sugar, so why don’t they grow in
honey? Calculate the osmotic pressure of honey to
find out. Osmotic pressure of bacteria ~ 8 - 30 atm.
Composition of sugars: 70% fructose and glucose
(C6H12O6)
30% maltose and sucrose
(C12H22O11)
Density of honey: 1.40 g/mL
Example
A 25 mL aqueous solution containing 0.420 g of
hemoglobin has an osmotic pressure of 4.6 torr
at 27°C. What is the molar mass of
hemoglobin?
1atm = 6.05 10 3 atm
= ( 4.6torr ) 760torr 1L = 0.025mL
V = ( 25mL ) 1000mL T = 300K
n
= RT
V V
n=
RT
6.05 10 3 atm mol K
n=
( 0.025L )
300
K
0.08206
L
atm
n=
0.420g
molar mass of hemoglobin = 6.14 10 6 mol molar mass of hemoglobin =
Answer: 6.84 104 g/mol
Sheets
Page 13
Lecture 20
COLLOIDS
small
true
solution
molecules
SIZE
colloidal
suspension
2–2000 nm
MILK
(fat particles)
FOG
(water droplets)
Phases mutually insoluble
hydrophillic vs. hydrophobic colloids
large
mixture
particles
influenced
by gravity
river silt
WHY IS THE SKY BLUE?
Light passes thru solns without scattering
Light passes thru colloidal suspensions
with scattering (milk, fog)
Tyndall Effect - particles scatter light of
λ about the same as their size
atoms
molecules
~ nm, scatter x-rays
colloids up to ~hundreds of nm’s
(visible light is 400-700 nm)
So…colloids scatter light
Scatter blue more effectively than red
LIGHT SCATTERING
…or…
WHY IS THE SKY BLUE?
sun
s un
colloids scatter more blue than red
Hydrophilic vs. hydrophobic colloids
• hydrophilic: water-loving
• hydrophobic: water-fearing
• (water-soluble) proteins: hydrophobic core
with hydrophilic surface
• detergents:
hydrophobic tail
with hydrophilic
head
Sheets
Page 16
Lecture 20