Question

For each of the following compounds, decide whether the compound's solubility in aqueous solution changes with pH. If the solubility does change, pick the pH at which you'd expect the highest solubility.
a. Ca(OH)2
b. CuBr
c. Ca3(PO4)2

Answer 1

Answer:

1. Ca(OH)₂ : Yes

Highest solubility  = pH of 2

2. Cu Br : Yes

Highest solubility  = pH of 4

3. Ca₃(PO₄)₂ : Yes

Highest solubility = pH of 2

Explanation:

From the common ion effect, the solubility of an ionic compound in a solution which already contains one of the ions in that compound will reduce. This is because, when an external stress is placed on a system in equilibrium,  the equilibrium position will shift to remove the effect of that stress. Therefore, when more of the same ions are added to a solution already containing that ion, equilibrium will shift towards removal of the ion, thereby reducing solubility.

1. For Ca(OH)₂:

Ca(OH)₂ is a fairly soluble ionic compound whose dissociation equation is given below:

Ca(OH)₂ (s) ----> Ca²+ (aq) + 2 OH- (aq)

Increase in pH will result in addition of more OH- ions, therefore, its solubility will decrease. On the other hand, a decrease in pH will remove OH- ions, thereby increasing solubility. The pH of highest solubility is 2.

2. For CuBr:

CuBr is a slightly acidic salt as it is salt of a strong acid, HBr and a weak base Cu(OH)₂. Hydrolysis of the salt will result in an acidic medium:

2 CuBr (s) + 2 H₂O (l) ----> 2 HBr (aq) + Cu(OH)₂ (aq).

Thus, an increase in pH will result in an increase in the solubility of CuBr due to the removal of H+ ions. The highest solubility will be at pH of 4.

3) For Ca₃(PO₄)₂

Calcium phosphate is a salt of slightly basic salt as it is a salt of a weak acid, H₃PO₄ and stronger base Ca(OH)₂.

Hydrolysis of the salt will result in a basic medium:

Ca₃(PO₄)₂ (s) + 6 H₂O (l) ---> 3 Ca(OH)₂ (aq) + 2 H₃PO₄ (aq)

Therefore, its solubility increases with decrease in pH due to the removal of OH- ions. The highest solubility will be at pH of 2.