Chap 19C - Solubility

When NH3 is gradually added to a solution of Zn2+ ions, a white precipitate is first formed. The white precipitate then dissolves on further addition of NH3 to form a colourless solution. Explain.  

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq) ----- (1)

Zn2+(aq) + 2OH–(aq) ⇌ Zn(OH)2(s) ----- (2)

Zn2+(aq) + 4NH3(aq) ⇌ [Zn(NH3)4]2+(aq) ----- (3)

• NH3 partially dissociates in water to produce OH- 

• In addition to NH3(aq), [OH–] increases, this causes the ionic product [Zn2+][OH–]2 to be greater than Ksp.

• POE (2) shifts right to counteract the increase in [OH–] -> White precipitate of Zn(OH)2 is observed.

• On further addition of NH3(aq), [NH3] increases and POE (3) shifts right to form colourless complex, [Zn(NH3)4]2+ -> decrease in [Zn2+]

• POE (2) shifts left to counteract the decrease in [Zn2+]

• When the ionic product, [Zn2+][OH–]2 is smaller than its Ksp, the white precipitate of Zn(OH)2 dissolves to give a colourless solution

State complex ion and equilibrium equation for AI3+, Ag+

State complex ion and equilibrium equation for Cu2+, Cr3+

State complex ion and equilibrium equation for Zn2+ (both)

State which type of salt is affected by pH

• Solubility of a sparingly soluble salt will be affected by pH changes if the anion of the salt is at least moderately basic

  • Solubility increases if solution becomes more acidic 

Describe solubility of Ca3(PO4)2 in acid 

Ca3(PO4)2 (s) ⇌ 3Ca2+ (aq) + 2PO43– (aq) ----- (1)

HCl (aq) → Cl– (aq) + H+ (aq) ----- (2)

H+(aq) + PO43– (aq) ⇌ HPO42– (aq) ----- (3)

• PO43- is a conjugate base of a weak acid HPO42- -> PO43- undergoes hydrolysis 

• The H+ formed from the dissociation of HCl reacts with PO43– in equilibrium (3).

• [PO43–] decreases and POE (1) shifts right to counteract the decrease in [PO43–]

• When the ionic product [Ca2+]3[PO43–]2 is smaller than its Ksp, the Ca3(PO4)2 precipitate dissolves, resulting in an increase in solubility of Ca3(PO4)2 in acidic solution (low pH)

Describe solubility of Ca3(PO4)2 in base

Ca3(PO4)2 (s) ⇌ 3Ca2+ (aq) + 2PO43– (aq) ----- (1)

PO43- + H2O ⇌ HPO42- + OH- ----- (2)

• PO43- is a conjugate base of a weak acid HPO42- -> PO43- undergoes hydrolysis 

• In basic solution, [OH-] is high -> POE in (2) shifts left to counteract increase in [OH-] -> [PO43-] increases 

• Ionic product [Ca2+][PO43-]2 > Ksp -> POE in (1) shifts left to counteract increase in [PO43-]

• More Ca3(PO4)2 solid will be precipitated -> decrease in solubility in basic solution