CHAPTER 16 NOTES
SOLUBILITY EQUILIBRIA
16.1 Solubility Equilibria and the Solubility Product
In this section we are going to discuss the equilbria associated with solids dissolving to form aqueous solutions.
We assume that ionic solids DISSOCIATE in water.
EXAMPLE – dissociation of calcium fluoride.
CaF2(s)  Ca2+(aq) + 2 F-(aq)
When the salt is first added there can only be a forward reaction – as soon as calcium and fluoride ion appear, a
reverse reaction can occur – and an equilibrium is established as shown below.
CaF2(s)  Ca2+(aq) + 2 F-(aq)
NOTE:
When a salt dissociates and establishes an equilibrium, the solution is said to be SATURATED. You can visually
establish that the solution is saturated as you will see undissolved solute present.
We can write the equilibrium expression, Ksp, according to the Law of Mass Action for the dissociation of calcium
fluoride as
Ksp =
The constant Ksp is called the solubility product constant or simply the solubility product for the equilibrium
expression.
EXAMPLE:
Show bismuth sulfide Bi2S3 dissociating in water and write its Ksp expression.
TABLE 16.1 ( p. 761) Ksp values at 25oC for common ionic solids.
CALCULATING Ksp FROM SOLUBILITY DATA
EXAMPLE #1
Copper(I) bromide has a measured solubility of 2.0 x 10-4 mol L-1 at 25oC. Calculate its Ksp value.
EXAMPLE #2
Calculate the Ksp value for bismuth sulfide (Bi2S3), which has a solubility of 1.0 x 10-15 mol L-1 at 25oC.
CALCULATING SOLUBILITY FROM GIVEN Ksp
EXAMPLE
The Ksp of copper(II) iodate, Cu(IO3)2, is 1.4 x 10-7 at 25oC. Calculate its solubility at 25oC.
RELATIVE SOLUBILITIES
Can we tell simply by looking at Ksp values for two salts which salt has a greater molar solubility than the other?
It depends!!!
#1. The salts being compared produce the same number of ions in solution. For example:
AgI, CuI, and CaSO4 all produce 2 ions each in solution.
The Ksp for CuI is 5.0 x 10-12
The Ksp for AgI is 1.5 x 10-16
The Ksp for CaSO4 is 6.1 x 10-5
Salts that produce the same number of ions in solution can have their solubilities compared by simply comparing
the Ksp values. The larger the Ksp, the more soluble the salt.
Rank the above salts according to increasing solubility.
#2. The salts being compared produce different numbers of ions when they dissociate. For example:
CuS produces _____ ions in solution.
Ag2S produces _____ ions in solution.
Bi2S3 produces _____ ions in solution.
In this situation, Ksp values cannot simply be compared to determine relative salt solubility. We would have to
calculate the molar solubility of each salt – as in the previous exercise – and then rank them.
THE COMMON ION EFFECT
So far we have considered ionic salts dissociating in pure water. We will now see what happens when the water
already contains an ion in common with the salt that is dissociating.
EXAMPLE
Calculate the solubility of solid CaF2 (Ksp = 4.0 x 10-11) in
(a) pure water
(b) 0.025 M NaF solution.
Conclusions:
1) Solubility ________________ with a common ion present.
2) According to Le Chatelier’s Principle – adding a common ion shifts the equilibrium to the ________.
pH and SOLUBILITY
The pH of a solution can greatly affect a salt’s solubility.
Example:
Consider the solubility of magnesium hydroxide:
How would an decrease in pH affect the solubility? Increase in pH?
For salts, the general rule is that is the anion X- is an effective base - that is, if HX is a weak acid – the salt MX
shows an increased solubility in an acidic solution.
Example:
Which of the following salts will be more soluble in an acidic solutions:
(a) Ag3PO4
(b) AgCl
(c) BaF2
(d) CaCO3
16.2 PRECIPITATION AND QUALITATIVE ANALYSIS
So far we have considered solids dissolving in solutions. Now we will consider the reverse process – the formation
of a solid from solution.
We will predict whether a precipitate will form when 2 solutions are mixed. We will use the
ion product, Q, which is defined just like the Ksp expression except initial concentrations are used instead of
equilibrium concentrations.
To predict whether precipitation will occur, we consider the relationship between Q and Ksp.
If Q = Ksp, equilibrium exists (saturated solution)
If Q > Ksp, precipitation occurs until Q = Ksp
If Q < Ksp, no precipitation occurs. Solid will dissolve until Q = Ksp
Example Problem:
a) A solution is prepared by adding 750. mL of 4.00 x 10-3 M Ce(NO3)3 to 300.0 mL
of 2.00 x 10-3 M KIO3. Will Ce(IO3)3 (Ksp = 1.9 x 10-10) precipitate from the
solution?
SELECTIVE PRECIPITATION
Mixtures of metal ions in aqueous solution are often separated by selective precipitation, that is, by using a reagent
whose anion forms a precipitate with only one or a few of the metal ions in the mixture.
EXAMPLE #1:
Suppose we have a solution containing both Ba2+ and Ag+ ions. If NaCl is added to the solution,
________precipitates as a white solid but since _______ is soluble, the _______ ions remain in solution.
EXAMPLE #2:
A solution contains 1.0 x 10-4 M CuNO3 and 2.0 x 10-3 M Pb(NO3)2. If 1.0 M NaI is added gradually to this
solution, will PbI2 (Ksp = 1.4 x 10-8) or CuI (Ksp = 5.3 x 10-12) precipitate first? Specify the concentration of Inecessary to begin precipitation of each salt.
HOMEWORK (on separate sheet)
2/18 Homework - Ksp
Write the dissociation equations for the salts in all the following questions!
1. The solubility of silver sulfide is 3.4 x 10-17 mol L-1 at 25oC, calculate the Ksp.
2. The solubility of lead (II) bromide is at 25oC is 1.0 x 10-2 mol/L. Calculate Ksp.
3. The solubility of ferrous hydroxide is 6.88 x 10-7 g per 100. mL at 25oC. (Hint – must convert solubility to
mol/L) Calculate the Ksp.
4. The Ksp for lead (II) sulfide is 7.0 x 10-29. Calculate the solubility of this compound.
2/19 Homework - Solubility
1. For each of the following pairs of solids, determine the solid with the smallest molar solubility:
(a) calcium fluoride (Ksp = 4.0 x 10-11) or barium fluoride (Ksp = 2.4 x 10-5)
(b) calcium phosphate (Ksp = 1.3 x 10-22) or ferric phosphate (Ksp = 1.0 x 10-22)
2. Calculate the solubility of AgBr at 25oC in (a) pure water
(Ksp for AgBr at 25oC = 5.0 x 10-13)
(b) 3.0 x 10-2 M AgNO3
3. Which of the following salts will be substantially more soluble in an acidic solution than in pure water?
(a) CuBr
(b) AgCN
(c) BrI3
(d) ZnS
2/20 Homework - Qsp
1. Will a precipitate form when 0.10 L of 8.0 x 10-3 M Pb(NO3)2 is added to 0.40 L of 5.0 x 10-3 M Na2SO4?
(Ksp for lead II sulfate is 1.3 x 10-8)
2. A 50.0 mL solution of 6.0 x 10-3M CaCl2 is mixed with 30.0 mL of 4.0 x 10-4 M NaF solution. Will precipitation
occur? (Ksp for calcium fluoride is 4.0 x 10-11)
3. If 0.200 M Na2S is added slowly to a beaker containing both 0.150 M AgNO3 and 0.200 M Ni(NO3)2 at 25oC,
which will precipitate first, AgcS (s) (Ksp = 1.6 x 10-49) or NiS (s) (Ksp = 3 x 10-21). Show calculations to
support your answer.
2/21 Homework
Complete AP Problems given in class