Question

At what pH should precipitate Ni(OH)2 from 1 solution containing ions Ni2+ with concentration
0.2 M?
KPT Mg(OH)2=5×10-¹⁶

Answer 1

I wonder why you want to know what pH will ppt Ni(OH)2 but you give the Ksp for Mg(OH)2. Here is how you do the Ni(OH)2. I will pick up Ksp for Ni(OH)2 from the internet but make sure you use the Ksp available in your classroom literature. I find Ksp Ni(OH)2 = 2 x 10^-16.
..................Ni(OH)2 (s) ==> Ni^2+ + 2OH^-
Ksp = (Ni^2+)(OH)^2 The problem states (Ni^2+) = 0.2 M. Substitute Ni and Ksp into the equation to obtain 2E-16 = (0.2)(OH^-)^2
(OH^-) = sqrt [(2E-16/0.2)] = ?
Convert OH^- to pOH by pOH = -log(OH^-), then
you know pH + pOH = 14. Solve for pH. Post your work if you get stuck.