Solubility
Solubility
“Insoluble” salts are governed by equilibrium
reactions, and are really sparingly soluble. There
is a dynamic equilibrium between the solvated
ions and the crystalline solid.
Solubility
Solubility
The extent to which a substance dissolves is
its solubility. Solubility, as with all equilibria, is
highly temperature dependent.
Solubility may be expressed as grams of
compound/100 mL of H2O, or as molarity, M.
Solubility Conventions
The equilibrium constants for solubility are
based on the insoluble solid appearing on the
left side of the reaction, with the solvated ions
on the right side.
PbI2(s) ↔ Pb2+(aq) + 2I-(aq)
Ksp = [Pb2+][ I- ]2 = 1.4 x 10-8 @25oC
sp stands for solubility product.
Solubility
The molar solubility can be calculated from
the value of Ksp (and vice versa).
Since salts have differing numbers of cations
and anions, you cannot look at a table of Ksp
values and determine which compounds have
the greater solubility.
Ksp Values
Problem

Calcium phosphate has a Ksp of 1.3 x 10-32.
Calculate the molar solubility of calcium
phosphate.
Predicting Precipitation Reactions
If the ions which may form a precipitate
come from two different sources (the cation
comes from one soluble salt, the anion from
another), calculate the value of Q.
Predicting Precipitation Reactions

Will a precipitate form if 10.0 mL of 0.010M
AgNO3 is added to 20.0 mL of 0.10M
Na2SO4?
1.
Determine the “insoluble” product.
Calculate the concentration of each ion of the
“insoluble” product.
Calculate Q and compare the value to Ksp.
2.
3.
pH and Solubility
Since most anions are weak bases, solubility
is highly pH dependent. The basic anion may
become protonated in acidic solutions, and thus
the “insoluble” salt will dissolve.
pH and Solubility
For example, consider the solubility of CuS.
Ksp=8.5 x 10-45. Sufide ion is the conjugate base
of HS-, a weak acid with a Ka of 1.3 x 10-13.
This means that sulfide ion is a relatively strong
weak base, and will accept protons.
pH and Solubility
Solubility Reaction:
CuS(s) ↔ Cu2+(aq) + S2-(aq) Ksp= 8.5 x 10-45
Reaction of Sulfide with Acid:
S2-(aq) + H3O+(aq) ↔ HS-(aq) + H2O(l)
Kb = Kw/Ka = (1.0 x 10-14)/1.3 x 10-13 = 7.7 x 10-2
Since sulfide is basic, CuS will dissolve in acidic
solution.
pH and Solubility
Since sulfide is basic, CuS will dissolve in
acidic solution. As acid is added, this reaction is
shifted towards the right.
S2-(aq) + H3O+(aq) ↔ HS-(aq) + H2O(l)
pH and Solubility
Since sulfide is basic, CuS will dissolve in
acidic solution. As acid is added, this reaction is
shifted towards the right.
S2-(aq) + H3O+(aq) ↔ HS-(aq) + H2O(l)
pH and Solubility
Since sulfide is basic, CuS will dissolve in
acidic solution. As acid is added, this reaction is
shifted towards the right.
S2-(aq) + H3O+(aq) ↔ HS-(aq) + H2O(l)
The decrease in [S2-] shifts this reaction
toward the right so as to make more sulfide.
CuS(s) ↔ Cu2+(aq) + S2-(aq)
pH and Solubility
CuS(s) ↔ Cu2+(aq) + S2-(aq)
The result is a large increase in solubility of
CuS in acidic solutions.
Problem: pH and Solubility

Which of the following “insoluble” salts will be
more soluble in acidic solutions rather than pure
water? Support your answer with chemical
equations.
AgCl, Fe(OH)3, BaSO4, CaCO3, PbBr2
Selective Precipitation
Mixtures of metal ions in aqueous solution
are often separated by selective precipitation. A
solution containing an anion that will only
precipitate one of the metals while leaving the
others in solution is used.