Question

You are investigating the effect of strong base on the solubility of zinc hydroxide, Zn(OH)2 (s).
(Ksp= 3 x 10- 15 for Zn(OH)2 (s))
You prepare 1.0 L solution of 0.10M NaOH and add excess zinc hydroxide, Zn(OH)2 (s).
Instead of suppresing the solubility, you observe extra solubility.
Your research advisor tells you that zinc ions react with four hydroxide ions in a solution to form the complex ion Zn(OH)42- (aq).
(Kf = 2 x 10 15 for Zn(OH)42- (aq))
Calculate the pH of your solution.

Answer 1

I would do this.
Zn(OH)2 ==> Zn^2+ 2OH Ksp = ?
Zn^2+ + 4OH^- ==> [Zn(OH)4]^2- Kf=?
Add these two equations.
Zn(OH)2 + 2OH^- ==> [Zn(OH)4]^2-
.solid....+2x.........-x
Krxn = Ksp*Kf = approx 6
Then [(x)/(2x)^2] = 6
Solve for x and convert to pH.