Updates
• Midterms are Feb. 08 and Mar. 15 at 7pm…
anyone with a night class or other midterm will
write it at 6 pm (notify me at least 1 week prior if
you have one of these conflicts)
• Assignment 01 is posted and is due (in class)
Wed., January 24
Physical Properties
of Solutions
Chapter 13
Effect of pressure on gas
solubility
•
The solubilities of solids and
liquids are not affected
appreciably by pressure
•
When the pressure of a gas is
increased, as in (b), the rate at
which gas molecules enter the
solution increases
•
The concentration of solute
molecules at equilibrium
increases in proportion to the
pressure
•
So the solubility of a gas
increases with pressure
Pressure and Solubility of Gases
Solubility decreases as pressure decreases
•
Soft drink bottled under
CO2 pressure greater
than 1 atm
•
When the bottle is
opened, partial
pressure of CO2 above
the solution decreases
•
Solubility of CO2
decreases -> bubbles
13.5
Pressure and Solubility of Gases
Solubility decreases as pressure decreases
In many consumer beverages such as soft drinks, carbonation is used to give "bite".
Contrary to popular belief, the fizzy taste is caused by the dilute carbonic acid
inducing a slight burning sensation, and is not caused by the presence of bubbles.
This can be shown by drinking a fizzy drink in a hyperbaric chamber at the same
pressure as the beverage. This gives much the same taste, but the bubbles are
completely absent.
13.5
Pressure and Solubility of Gases
The solubility of a gas in a liquid is proportional to the
pressure of the gas over the solution (Henry’s law).
c = kP
c is the concentration (M) of the dissolved gas
P is the pressure of the gas over the solution
k is a constant (mol/L•atm) that depends only on temperature
low P
high P
low c
high c
13.5
SAMPLE EXERCISE
A Henry’s Law Calculation
Calculate the concentration of CO2 in a soft drink that is bottled with a partial pressure of CO 2 of 4.0 atm over
the liquid at 25°C. The Henry’s law constant for CO2 in water at this temperature is 3.1  10–2 mol/L-atm.
Solution
Analyze: We are given the partial pressure of CO2, and the Henry’s law constant, k, and asked to calculate the
concentration of CO2 in the solution.
Plan: With the information given, we can use Henry’s law to calculate the solubility.
Solve:
2
Check: The units are correct for solubility, and the answer has two significant figures consistent with both the
partial pressure of CO2 and the value of Henry’s constant.
PRACTICE EXERCISE
Calculate the concentration of CO2 in a soft drink after the bottle is opened and equilibrates at 25°C under a CO2
partial pressure of 3.0  10–4 atm.
Blood gases and deep sea diving
•
•
•
•
•
Solubility increases as pressure increases
Divers who use compressed gases must be concerned about the solubility of the gases in
their blood
At depth, the blood contains higher concentrations of dissolved gases
Ascension, if too rapid, will cause the blood to fizz similar to 7-UP when opened!
This is called decompression sickness, or “the bends”, which is painful and can be fatal
because the bubbles affect things like nerve impulses
Temperature and Solubility
Solubility of most solid solutes in water
increases with increasing temperature
In contrast, solubility of gases in water
decreases with increasing temperature
13.4
Temperature and Solubility
• Carbonated beverages go
“flat” as they warm due to a
decreased solubility of
dissolved CO2
In contrast, solubility of gases in water
decreases with increasing temperature
• Bubbles form on the inside
wall of a cooking pot when
water is heated even though
the temperature is well below
boiling
• Thermal pollution of lakes
and streams causes low
oxygen levels in deeper
layers
13.4
Colligative Properties
• Changes in colligative properties
depend only on the number of solute
particles present, not on the identity of
the solute particles.
• Among colligative properties are
Vapor pressure lowering
Boiling point elevation
Melting point depression
Osmotic pressure
Vapor Pressure
Because of solute-solvent
intermolecular attraction,
higher concentrations of
nonvolatile solutes make it
harder for solvent to escape
to the vapor phase.
Colligative Properties of Nonelectrolyte Solutions
Nonvolatile solutes lower the vapor pressure of a solvent
by an amount proportional to the solute mole fraction.
Vapor-Pressure Lowering
P1 = X1 P
0
1
Raoult’s law
P 10 = vapor pressure of pure solvent
X1 = mole fraction of the solvent
If the solution contains only one solute:
X1 = 1 – X2
P 10 - P1 = DP = X2 P 10
X2 = mole fraction of the solute
13.6
SAMPLE EXERCISE Calculation of Vapor-Pressure Lowering
Glycerin (C3H8O3) is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25°C. Calculate the vapor
pressure at 25°C of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure of
pure water at 25°C is 23.8 torr.
Solution
Analyze: Our goal is to calculate the vapor pressure of a solution, given the volumes of solute and solvent and
the density of the solute.
Plan: We can use Raoult’s law to calculate the vapor pressure of a solution. The mole fraction of the solvent in
the solution, XA, is the ratio of the number of moles of solvent (H2O) to total solution (moles C3H8O3 + moles
H2O).
Solve: To calculate the mole fraction of water in the solution, we must determine the number of moles of
C3H8O3 and H2O:
SAMPLE EXERCISE continued
We now use Raoult’s law to calculate the vapor pressure of water for the solution:
The vapor pressure of the solution has been lowered by 0.6 torr relative to that of pure water.
PRACTICE EXERCISE
The vapor pressure of pure water at 110°C is 1070 torr. A solution of ethylene glycol and water has a vapor
pressure of 1.00 atm at 110°C. Assuming that Raoult’s law is obeyed, what is the mole fraction of ethylene
glycol in the solution?
Answer: 0.290
If both components of the solution are volatile, the vapor
pressure of the solution is the sum of the individual partial
pressures.
PA = XA P A0
PB = XB P 0B
PT = PA + PB
PT = XA P A0 + XB P 0B
Pure toluene
Pure benzene
1:1 mix
13.6
Boiling-Point Elevation
DTb = Tb – T b0
T b0 is the boiling point of
the pure solvent
T b is the boiling point of
the solution
Tb > T b0
DTb > 0
DTb = Kb m
m is the molality of the solution
Kb is the molal boiling-point
elevation constant (0C/m)
13.6
Freezing-Point Depression
DTf = T 0f – Tf
T
0
Tf
f
is the freezing point of
the pure solvent
is the freezing point of
the solution
T 0f > Tf
DTf > 0
DTf = Kf m
m is the molality of the solution
Kf is the molal freezing-point
depression constant (0C/m)
13.6
13.6
What is the freezing point of a solution containing 478 g
of ethylene glycol (antifreeze) in 3202 g of water? The
molar mass of ethylene glycol is 62.01 g.
DTf = Kf m
Kf water = 1.86 0C/m
moles of solute
m =
mass of solvent (kg)
478 g x
1 mol
62.01 g
=
= 2.41 m
3.202 kg solvent
DTf = Kf m = 1.86 0C/m x 2.41 m = 4.48 0C
DTf = T 0f – Tf
Tf = T 0f – DTf = 0.00 0C – 4.48 0C = -4.48 0C
13.6
Osmotic Pressure (p)
Osmosis is the selective passage of solvent molecules through a porous
membrane from a dilute solution to a more concentrated one.
A semipermeable membrane allows the passage of solvent molecules but
blocks the passage of solute molecules.
Osmotic pressure (p) is the pressure required to stop osmosis.
dilute
more
concentrated
13.6
Osmotic Pressure (p)
High
P
Low
P
p = MRT
M is the molarity of the solution
R is the gas constant
T is the temperature (in K)
13.6
A cell in an:
isotonic
solution
hypotonic
solution
hypertonic
solution
13.6
Colligative Properties of Nonelectrolyte Solutions
Colligative properties are properties that depend only on the
number of solute particles in solution and not on the nature of
the solute particles.
Vapor-Pressure Lowering
P1 = X1 P 10
Boiling-Point Elevation
DTb = Kb m
Freezing-Point Depression
DTf = Kf m
Osmotic Pressure (p)
p = MRT
13.6
Colligative Properties of Electrolyte Solutions
0.1 m Na+ ions & 0.1 m Cl- ions
0.1 m NaCl solution
Colligative properties are properties that depend only on the
number of solute particles in solution and not on the nature of
the solute particles.
0.1 m NaCl solution
van’t Hoff factor (i) =
0.2 m ions in solution
actual number of particles in soln after dissociation
number of formula units initially dissolved in soln
i should be
nonelectrolytes
NaCl
CaCl2
1
2
3
13.7
Colligative Properties of Electrolyte Solutions
Boiling-Point Elevation
DTb = i Kb m
Freezing-Point Depression
DTf = i Kf m
Osmotic Pressure (p)
p = iMRT
13.7