Calculating pH for Mg(OH)2 Precipitation 0.1M Mg2+

[zorro]

Homework Statement

Caculate the pH at which Magnesium Hydroxide begins to precipitate
from a solution containing 0.1M Mg2+ ion? Ksp=10^-11

Homework Equations

The Attempt at a Solution

I did not understand why Ksp is given.
Since the concentration of Mg2+ is 0.1, concentration of OH- will be 0.2 (by stoichiometry).
Moreover there is no other source of OH-.
Where am I wrong?

 

[Borek]

You start with a solution that contains Mg²⁺ (from whatever source) and you add base till precipitation starts.

Your ability to understand questions in unintended way is incredible

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[zorro]

LOL. May be it is because I think too much behind a problem