General Chemistry Principles, Patterns, and Applications, 2011

Equation 17.13
MA s
( ) M + ( aq) + A - aq
( )Ksp = [M+][A-]
The anion can also react with water in a hydrolysis reaction:
Equation 17.14
( ) + H2O( l) OH - ( aq) + HA( aq)
A - aq
Because of the reaction described in Equation 17.14, the predicted solubility of a sparingly soluble salt that
has a basic anion such as S 2− , PO4 3− , or CO3 2− is increased, as described in Section 17.1 "Determining the
Solubility of Ionic Compounds". If instead a strong acid is added to the solution, the added H + will react
essentially completely with A − to form HA. This reaction decreases [A − ], which decreases the magnitude of
the ion product (Q = [M + ][A − ]). According to Le Châtelier’s principle, more MA will dissolve until Q = Ksp.
Hence an acidic pH dramatically increases the solubility of virtually all sparingly soluble salts whose
anion is the conjugate base of a weak acid. In contrast, pH has little to no effect on the solubility of salts
whose anion is the conjugate base of a stronger weak acid or a strong acid, respectively (e.g., chlorides,
bromides, iodides, and sulfates). For example, the hydroxide salt Mg(OH)2 is relatively insoluble in water:
Equation 17.15
Mg( OH )2 s
( ) Mg2 + ( aq) + 2OH - aq
( )Ksp = 5.61´10 -12
When acid is added to a saturated solution that contains excess solid Mg(OH)2, the following reaction
occurs, removing OH − from solution:
Equation 17.16
( ) +OH -( aq) ® H2O( l)
H + aq
The overall equation for the reaction of Mg(OH)2 with acid is thus
Equation 17.17
Mg( OH )2 s
( ) + 2H + ( aq) Mg2 + ( aq) + 2H2O( l)
As more acid is added to a suspension of Mg(OH)2, the equilibrium shown in Equation 17.17 is driven to
the right, so more Mg(OH)2 dissolves.
Such pH-dependent solubility is not restricted to salts that contain anions derived from water. For
example, CaF2 is a sparingly soluble salt:
Equation 17.18
CaF2 s
( ) Ca2 + ( aq) + 2F - aq
( )Ksp = 3.45 ´10 -11
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