Equilibrium of a Sparingly Soluble Ionic Compound

[daphnelee-mh]

Homework Statement

I can't get the correct answer, isn't it if Ksp>[Mn2+][OH-]^2, there is no precipitation. How to get the specific concentration of [OH-]?

Relevant Equations

Kb = [NH4+][OH-]/[NH3]
Ksp=[Mn2+][OH-]^2

Answer given is 6.61g.

 

[Borek]

You are looking for the highest concentration of OH^- that doesn't produce precipitation, don't you?

 

[daphnelee-mh]

May I know which step is wrong? thanks

 

[mjc123]

You have worked out the concentration in g/L. What is the mass in the actual solution?

 

[daphnelee-mh]

Is this correct ? thanks.

 

[mjc123]

It is not quite equal to the given answer; I think that's because you rounded off [OH^-] to 1.4e-6 M instead of 1.435e-6 M. Keep the extra digits through the calculation, and round off to the appropriate sig figs at the end.