Question

Calculate the molar solubility of Mg(OH)2 in the following solvents:
(i) Pure Water
(ii) 8.60×10−2M MgCl2
(iii) 3.70×10−2M KOH(aq)


How should i solve this problem?

Answer 1

In pure H2O:
.........Mg(OH)2(s) ==> Mg^2+ + 2OH^-
I.........solid.......0...........0
C..........-x.........x....,,,....2x
E.........solid-x.....x...........2x

Substitute the E line into Ksp expression and solve for x = solubility Mg(OH)2 in mols/L.
NOTE: Mg(OH)2(s) is NOT included in the Ksp expression; therefore, solid-x will not appear.

Answer 2

What is the value of Ksp?

Answer 3

I don't know. Look it up in your text/notes or on the web.
By the way, part 2 and 3 of the problem are common ion problems and each will decrease the solubility; i.e., solubility of parts 2 and 3 will be less than the solubility of part 1.