Question

A solution is 5 mM in each of the following ions:

number	ion	Ksp of M(OH)2
1	Mg2+	1.8e-11
2	Cd2+	2.5e-14
3	Co2+	1.6e-15
4	Zn2+	4.4e-17
5	Cu2+	2.2e-20

Indicate which of the metal ions would precipitate (or start to precipitate) at each of the following pH values. Indicate your answer with the number of the ion. Use 0 to indicate no precipitate. If more than one precipitate is expected, list the numbers in increasing order and separate them with commas. For example, 3,4,5 is ok but 5,4,3 is not.

pH = 6.00:

pH = 10.00:

What is the pH to the nearest 0.1 pH unit at which Co(OH)₂ begins to precipitate?

pH =

Answer 1

for solubility at pH5, for 5mM solutions of all, x = 0.005M
for Mg+2
pOH = 14 - 5 = 9
[OH-]= 10^-9 = 1 x 10^-9 M
QMg+2 = 0.005M x (1x10^-9)^2 = 5 x 10^-21, Q<Ksp, no ppt
no ppt for any of the ions

for solubility at pH10
at pH10, [OH] = 1x10^-4M
for Mg+2 = Q = 5x10^-11, Q>Ksp, ppt
ppt forms for all ions
Mg(OH)2 will ppt out at pH 10 but Q is very close to the Ksp value which when Q=Ksp, the solution is saturated with no ppt

for pH of Co(OH)2 ppt formation
Q = Ksp at saturation
1.6x10^-15 = (0.005) x [OH-]^2
[OH-] = 5.66 x 10^-7M
pOH = log[OH-] = 6.24 so pH = 7.75