Exercise 4 | Solubility and Precipitation Reactions

Calculate the pH of a saturated aqueous solution of Mg(OH)2 (s).


Data: Ksp (Mg(OH)2) = 5.6 x 10-12 3

Chemical equation:

Mg(OH)2 (s)  Mg2+ (aq) + 2 HO- (aq)


Pure water: the only source of Mg2+ and HO- is Mg(OH)2

From the reaction stoichiometry: [HO-] = 2 [Mg2+]


Ksp = [Mg2+][HO-]2

Ksp = [HO-]3/2

⇒ [HO-] = 2.2 x 10-4 M


pH = 14 – pOH = 14 + log [HO-]

⇒ pH = 10.35