AP Chemistry
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Date _________ Bell _____
17.4-17.7  Solubility Equilibria
NOTES
17.4 Solubility Equilibria
 A saturated solution has solid undissolved solute mixed in with the ions dissolved in the aqueous
solution. The solution and the ions in solution are in a state of equilibrium.
 The solubility product constant, or solubility product(Ksp), tells the extent to which a compound will
dissolve in water
1. The compound does not have to technically be “soluble” but if it is an ionic compound, it will
break up into ions in solution
*Appendix D Table D-3 (Pg 1063) lists the Ksp of some electrolytic compounds
 The reactant in a solubility product equilibrium expression is a solid and so the Ksp is equal to the
product concentrations raised to their coefficients
1. Remember to only use the substances marked (aq), which are the products
2. Hint: the coefficient of the ion in the balanced equation is equal to its subscript in the solid
reactants. For example, the equilibrium equation
CaF2(s) Ca2+(aq) + 2 F-(aq)
Has the equilibrium expression
Ksp = [Ca2+][F-]2
*Sample Exercise 17.9 (Pg 723) shows how to write the solubility product equilibrium expression
 Solubility and solubility product constant are different
1. Solubility is grams of solute per liter of solution (g/L) or molar solubility (mol/L)
2. Ksp tells how much of the solid dissolves to form a saturated solution and has no units
3. The smaller the Ksp, the lower the solubility of the salt
*Fig 17.14 (Pg 724) shows how solubility and Ksp are related
 Ksp can be calculated from the solubility and the concentration of ions in solution. You can also use
the ICEBox method to find the solubility if given the Ksp
1. You will have to consider the molar ratio from the balanced equation for both methods.
*Sample Exercise 17.10 (Pg 724) shows how to find the Ksp given the concentration of one the ions
*Sample Exercise 17.11 (Pg 725) shows how to find the concentration of the ions using the Ksp
17.5 Factors that Affect Solubility
 Temperature, other ions in solution, and the solvent used all affect solubility but we will look at 3
other factors as well:
1. Presence of common ions in solution
2. pH of the solution
3. presence of complexing agents in solution
 The solubility of a slightly soluble salt is reduced if another compound is in the solution that has the
same ion. For example, CaF2 becomes less soluble if NaF is added to the solution because more Fions are being introduced. This shifts the equilibrium toward the solid CaF2. Think about
LeChatlier’s Principle.
1. You can use the ICEBox method to determine the solubility of a solid if common ions are added.
You will use the concentration of the common ion solution as the cincentraio for the common
ion.
*Sample Exercise 17.12 shows how to determine the solubility of a compound given the concentration
of a solution containing the common ion
 pH of a solution affects the solubility of any substance with a basic anion, like OH-. For example, a
buffer can be added to change the pH, which changes the OH concentration in the solution. Since
Ksp is constant, the concentration of the other ion changes as well. This increase or decrease means
that the compound will dissolve until the new concentration is satisfied. The more basic the anion,
the more its solubility is affected by pH
1. The solubility of any compound containing OH- will __________________ as the pH decreases
(Ex: Mg(OH)2 )
2. The solubility of any basic anion, or the anion of a weak acid, will __________________ as the
pH decreases or the [H+] increases (Ex: CaF2 because F- is the conjugate base of the weak acid
HF; also containing ions like CO32-, PO43-, CN-, S2-)
3. Salts with anions of strong acids will not be affected by pH since they make bases that have
negligible basicity
Sample Exercise 17.13 shows how you can determine the effect of an acid on solubility of a salt.
 Metal ions can act like Lewis acids (electron pair acceptors) and connect with compounds that act as
Lewis bases (electron pair donors). For example, Ag+ reacts with aqueous ammonia to produce the
complex ion Ag(NH3)2+
1. A complex ion is an ion composed of a metal ion, particularly a transition metal, bonded with
Lewis bases
2. The Lewis bases are known as complexing agents because they cause salts to break up and
partner with them in order to form complex ions.
 Formation constant, Kf, of a complex ion is the same as other constants.
1. The larger the Kf, the more stable the complex ion
2. Dissociation constant is the complex ion dissociating back into is Lewis acid and base. It is
equal to the inverse of the formation constant (Kd = 1 / Kf)
*Table 17.1 (Pg 732) lists the formation constants of some complex ions
 Some metal oxides (MO) and metal hydroxides (MOH) don’t really dissolve in water but will
dissolve in a strong base or acid because they are actually acids or bases
1. Amphoteric oxides are oxides that act as acids or bases (Al2O3, Cr2O3, ZnO, SnO, etc.)
2. Amphoteric hydroxides are hydroxides that act as acids or bases (Al(OH)3, Zn(OH)2, etc.)
17.6 Precipitation and Separation of Ions
 The reaction quotient, Q, can be compared to Ksp to determine equilibrium and if a precipitate will
form.
1. Ksp > Q then solid will dissolve until Ksp = Q
2. Ksp = Q then at equilibrium and you have a saturated solution
3. Ksp < Q then precipitation occurs until Ksp = Q
17.7 Qualitative Analysis for Metallic Elements
 Qualitative analysis is determining if something does or does not exist, such as whether a metal ion
is present in solution or not.

Quantitative analysis is determining how much of something exists, such as once it is determined
that a metal ion is present in solution you can then figure out how much of it is there.
 We can test for the presence of ions in solution through 3 general test areas:
1. Separate ions into broad groups based on solubility
2. Individual ions within these groups are separated by dissolving ions in the group
3. Ions are identified through specific tests
 The most used tests analyzes ions by separating them into 5 groups
1. Insoluble chlorides: Ag+, Hg22+ and Pb2+ form insoluble chlorides so if you add HCl and a
precipitate is formed, it has to be one of these. If not, the solution moves on to step 2.
2. Acid-insoluble sulfides: Since the solution is acidic after adding HCl, you add H2S and the most
insoluble metal sulfides will form precipitates (CuS, Bi2S3, CdS, PbS, HgS, As2S3, Sb2S3, and
SnS2. If not, the solution moves on to Step 3.
3. Base-insoluble sulfides and hydroxides: The solution is made basic and (NH4)2S is added. More
soluble metal sulfides are then precipitated (SnS, NiS, CoS, and MnS) as well as some insoluble
metal hydroxides (Al(OH)2, Fe(OH)3, and Cr(OH)3). If not, the solution moves on to Step 4.
4. Insoluble phosphates: There are only Group 1A and 2A metals left in solution at this point so
adding (NH4)2HPO4 will cause all of the Group 2A to precipitate as phosphates (Mg3(PO4)2,
Ca3(PO4)2, Sr3(PO4)2, and Ba3(PO4)2)
5. Alkali metal ions and NH4+: Only Group 1A and ammonium ions are left in the solution and
specific tests, such as flame tests, will have to be performed for each ion
 For each group, you will have to perform tests to narrow down the specific ion present.
*Fig 17.23 (Pg 738) shows the flow diagram for qualitative analysis of a solution.
*Sample Integrative Exercise (Pg 738) involves several concepts to derive the pH of a solution. This is
very similar to an AP Exam FRQ.
Additional Notes: