Chemistry – Dr. May Notes
Solutions and Colligative Properties
What is a solution?
A solution is a homogeneous mixture (solid, liquid, or gas) having any substance
that is evenly dispersed throughout another substance.
A solution may appear to be a pure substance but is a mixture.
The substance that is the dissolving medium is the solvent. The solute is the
material that is dissolved.
The Process of Solvation.
The polar water molecule can surround positive and negative ions. These ions are
said to be hydrated. As the crystals of an ionic material are converted into hydrated ions,
the process of dissociation occurs.
Dissolving a solute in a solvent is called solvation. Like dissolves like means that
polar solvents dissolve polar solutes.
Molecular solvation occurs when the polar solvent molecules attract the polar
solute molecules.
When there is no limit to the solubility of one substance in another (up to 100%),
the materials are said to be miscible. Water and ethanol are examples of miscible
materials.
Materials that do not dissolve in each other, like oil and water, are said to be
immiscible.
Solubility
When a solvent can dissolve no more solute, the solution is said to be saturated.
If more solute can be dissolved in the solvent, the solution is said to be unsaturated.
Solubility does not have a precise meaning but as a rule means that a 0.1 Molar
solution can be prepared at 0oC.
Solubility is the amount of material needed to make a saturated solution at a given
temperature. It is usually expressed as grams of solute per 100 grams of solvent at 0oC (or
some other temperature like 24oC)
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Factors Affecting Solubility
Solubility increases with an increase in the temperature of the solution. When a
solution is saturated at elevated temperatures then cooled, crystals usually form. The
solution will hold less solute at cool temperatures. If crystals do not form, the solution is
said to be supersaturated.
Pressure will increase the solubility of gas in solvent. Carbon dioxide in soda is an
example. The gas escapes when pressure is decreased (the soda is opened).
Ionic Equations and Precipitation Reactions
Reactions in Solution.
AgNO3 + KCl 
AgCl  + KNO3
In the above reaction silver nitrate and potassium chloride are soluble in water.
When they react in aqueous solution, potassium nitrate and silver chloride are formed.
Silver chloride is insoluble in water and forms a precipitate. The solution contains the
following ions:
Ag+
K+
NO3
Cl
The only compounds possible are the reactants and the products. When the silver
chloride is formed, the insoluble precipitate removes Ag+ and Cl from the mixture.
Writing Net Ionic Equations
AgNO3
KCl
Ag+ + NO3
+
K+ +
 Ag+ + NO3
 K+ + Cl
Cl 
AgCl  +
K+ + NO3
The above equation is called an ionic equation. K+ and NO3 are called spectator
ions since they remain ions in solution. The net ionic equation is determined by canceling
out the ions that occur on both sides of the equation.
Ag+ + NO3
+
K+ +
Cl 
AgCl  +
Ag+ +
Cl 
AgCl 
2
K+ + NO3
Colligative Properties of Solutions
Boiling Point Elevation and Freezing Point Depression
Solutions have higher boiling points and lower freezing points than the pure
solvents. This is why adding salt (sodium chloride or calcium chloride) to snow and ice
causing melting. The freezing point is lowered.
These are called colligative properties. The ionization of salts also makes water
conduct electricity. Salts are called electrolytes.
The number of solute particles, not their size or whether they are ions or
molecules, determine how the solute will affect the boiling and freezing points of water.
Molality
Molality (m) is the number of moles of solute per kilogram of solvent.
Molarity (M) can be thought of as weight per volume while Molality (m) is
weight per weight.
To determine how many grams of barium chloride (BaCl2) you would need to
prepare a 0.40 m solution in 100 grams of water, first determine the molecular weight of
barium chloride: 208.3 g/mole.
Then we know that molality (m) = moles of BaCl2  kg of water.
0.40 = moles  0.100 kg
moles of BaCl2 = 0.04
grams = moles x MW
grams = 0.04 x 208.3 = 8.33 grams
Calculations Involving Colligative Properties
The formula to calculate the boiling point elevation of water is:
Tb = kb x m
Tb is the increase in the boiling point in oC
kb is a constant with the value of 0.51
m is molality of the solution in moles/kg of solvent
Tf = kf x m
Tf is the difference in freezing point between solution and pure solvent in oC
kf is a constant with the value of 1.86
m is molality of the solution in moles/kg of solvent
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Solvents other than water have different kb and kf values. The calculations work
the same regardless of solvent and solute.
Calculating Molar Mass
By using the above colligative relationships combined with the fact that molar mass
(molecular weight)(MW) is related to grams and moles, we can determine the molecular
weight of an unknown material by knowing the freezing point depression (Tb) caused
by a given amount of solute in a given amount of water.
36.0 grams of material are dissolved in 100 grams of water. The solution begins to
freeze at 3.72oC. What is the molar mass of the solute?
Tf = kf x m
m ‗ Tf
kf
moles of material ‗ 2.0
0.1 (100 grams)
‗ 3.72 ‗ 2.0 moles/kg solvent
1.86
‗ 0.20 moles
MW ‗ grams ‗ 36.0 ‗ 180 grams/mole (molar mass) (MW)
moles
0.20
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