Chapter 11
Properties of Solutions
Important Vocabulary
• Homogeneous means there is only
one phase (compositions do not vary)
• Ex: Kool Aid, air, steel
• Solute: Gets dissolved
• Solvent: Does the dissolving
• Solution: Homogeneous mixture
consisting of a solute and solvent
Dilute vs. Concentrated
• Can’t be used in calculations
• Molarity, mass percent, and mole fraction
can be used to show solution
concentrations
Molarity
• Moles of solute/liters of solution
• Represented by M
• Example: A solution was prepared by adding
5.84 g of formaldehyde, H2CO, to 100.0 g of
water. The final volume of the solution was
104.0 mL. Calculate the molarity.
• Answer: 1.87 M H2CO
Mass Percent
• Percent by mass of the solute in the solution
• Mass Percent = (mass of solute/mass of
solution) X 100%
• Example: A solution was prepared by adding
5.84 g of formaldehyde, H2CO, to 100.0 g of
water. The final volume of the solution was
104.0 mL. Calculate the mass percent.
• Answer: 5.52 % H2CO, 94.48% H2O
Mole Fraction
• Represented by X
• Moles of part/moles of solution X 100%
Mole Frac. A = XA = nA/(nA+nB)
• Example: A solution was prepared by adding
5.84 g of formaldehyde, H2CO, to 100.0 g of
water. The final volume of the solution was
104.0 mL. Calculate the mole fraction.
• Answer: XH2CO = 0.0338, XH2O = 0.9662
Molality
• Represented by m
• Moles of solute per kilogram of solvent
Molality = moles of solute/kilogram of solvent
• Example: A solution was prepared by adding
5.84 g of formaldehyde, H2CO, to 100.0 g of
water. The final volume of the solution was
104.0 mL. Calculate the molality.
• Answer: 1.94 m H2CO
Solubility
• Shows what will dissolve in what
• “Like dissolves like” = polar solvents will
dissolve polar/ionic solutes and nonpolar
solvents will dissolve nonpolar solutes
Factors Affecting Solubility
1. Structure
2. Pressure
3. Temperature
1. Structure Effects
• Polarity of solute/solvent (like dissolves like)
• Example: vitamins are fat-soluble and watersoluble
– Fat-soluble = nonpolar, hydrophobic (water-fearing),
build up/stored in fatty tissue, too much =
hypervitaminosis
– Water-soluble = polar, hydrophilic (water-loving),
extra are excreted by the body
2. Pressure Effects
• Doesn’t affect liquids/solids, but has a large
affect on gases
• Gas solubility increases as the partial
pressure of the gas above the solution
increases
Henry’s Law
• Shows relationship between gas pressure
and concentration of dissolved gas:
C = kP
• C = concentration of dissolved gas
• K = constant for particular solution
• P = partial pressure of gas above solution
• Works best with gases that don’t
dissociate in/react with solvent
Henry’s Law Example
• The solubility of O2 is 2.2 X 10-4 M at 0C
and 0.10 atm. Calculate the solubility of
O2 at 0C and 0.35 atm.
• Answer: 7.7 X 10-4 M O2
3. Temperature Effects
• For most solids, solubility increases
as temperature increases
• For most gases, solubility decreases
as temperature increases
–Thermal Pollution in lakes: increase in
temp. lowers dissolved oxygen
concentrations
Vapor Pressure of Solutions
• If a solution contains a nonvolitile (not
easily vaporized) solute, its vapor
pressure is LOWER than the pure
solvent.
• Shells of water solvation make it so
it’s harder for the solvent to vaporize
• Molecules that do not dissociate
(break up) in water (solvent) have
higher vapor pressures than ionic
compounds that do dissociate
• The decrease in a solution’s
vapor pressure is proportional to
the number of particles the solute
makes in solution.
Answer This…
• Which compound affects the vapor
pressure of a solution the least:
glucose, sodium chloride, or calcium
chloride?
• Solutions with covalent compounds >
Solutions with ionic compounds
Raoult’s Law
• Calculates the expected vapor pressure of a
solution based on the solute/solvent
Psoln = XsolventP0solvent
• Psoln = observed vapor pressure of solution
• Xsolvent = mole fraction of solvent
• P0solvent = vapor pressure of the pure solvent
Example
• Glycerin, C3H8O3, is a nonvolatile liquid. What
is the vapor pressure of a solution made by
adding 164 g of glycerin to 338 mL of H2O at
39.8C? The vapor pressure of pure water at
39.8C is 54.74 torr and its density is 0.992
g/mL.
• Answer: 50.0 torr
Example #2
• What is the vapor pressure of a solution made
by adding 52.9 g of CuCl2, a strong electrolyte,
to 800.0 mL of water at 52.0C? The vapor
pressure of water is 102.1 torr, and its density is
0.987 g/mL.
• Answer: 99.4 torr
Colligative Properties
• Depend on the number of
solute particles, NOT their
identity in an ideal solution
1. Boiling-Point Elevation
2. Freezing-Point Depression
3. Osmotic Pressure
1. Boiling-Point Elevation
• Review boiling point definition: when
vapor pressure = atmospheric
pressure
• When solute is added to solvent, it
lowers the vapor pressure.
• More kinetic energy must be added to
bring the solution to boiling
• Boiling point is HIGHER in solutions
than in pure solvents
• Antifreeze in car engines (solute)
makes it so car engines don’t boil
in high temperatures
• The more solute particles
dissolved, the higher the boiling
point (identity doesn’t matter)
Boiling-Point Elevation Calculation
• Change in boiling point ∆Tb is the
difference between the boiling point of the
solution and the pure solvent
• Unit: °C/m
• Calculated using
∆Tb = Kb X msolute
Kb is a molal boiling-point elevation constant
of the solvent found on pg. 517
Example
• What is the boiling point of a
solution containing 96.7g of
sucrose (C12H22O11) in 250.0g
water at 1atm?
• Answer: 100.579°C
2. Freezing-Point Depression
• When solute is present, the normal
molecular freezing pattern is disrupted
• This makes it so the solution has to
lose more kinetic energy (get colder)
in order to solidify
• Freezing point of the solution is
LOWER than that of the pure solvent
• The more solute particles
dissolved, the more the freezing
point decreases (identity doesn’t
matter)
• Sidewalk salt and car antifreeze
work this way
Freezing-Point Depression
Calculation
• Change in freezing point ∆Tf is the
difference between the freezing point of
the solution and the pure solvent
• Unit: °C/m
• Calculated using
∆Tf = Kf X msolute
Kf is a molal freezing-point depression
constant of the solvent found on pg. 517
Example
• Determine the freezing point of a
solution made by adding 27.5 g of
methanol (CH3OH) to 250.0 g of
water.
• Answer: -6.39°C
Example
• Find the boiling point of
a 1.50m solution of
calcium chloride, CaCl2
and water.
• Answer: 2.30°C so the
new boiling point would
be 102.30°C.