Question

For each of the following ionic compounds, state whether the solubility will increase, decrease, or remain unchanged as a solution at pH 7 is made acidic.

(a) PbI₂

(b) AgOH

(c) Ca₃ (PO₄)₂

Short answer

1. The change in pH does not affect this salt.
2. The change in pH increases its solubility.
3. The change in pH increases the solubility of this salt.

Step-by-step solution

Effect of pH on solubility

In the case of sparingly soluble salt, its solubility is sometimes affected by changing the pH of the solution. Its solubility alters if the salt contains an anion of a weak acid.

Subpart (a)

The given salt is PbI₂ .

The anion is I^- . It is a weak conjugate base of the strong acid HI.

Thus, a change in pH does not affect this salt.

Subpart (b)

The given salt is AgOH.

The anion OH^- is a strong conjugate base of the weak acid H₂O . So, the decrease in pH increases the acidic strength, and thus it reacts with the hydroxide ion.

Thus, a change in pH increases its solubility.

Subpart (c)

The given salt is Ca₃ (PO₄)₂ .

It consists of an anion (PO₄^3-) that is a strong conjugate base of the weak acid H₃PO₄ .

Thus, a change in pH increases the solubility of this salt.