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

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To calculate the pH at which magnesium hydroxide (Mg(OH)2) begins to precipitate from a 0.1M Mg2+ solution, the solubility product constant (Ksp) is crucial, as it helps determine the concentration of hydroxide ions (OH-) needed for precipitation. The Ksp value of 10^-11 indicates that the product of the concentrations of Mg2+ and OH- must not exceed this value for the solution to remain unsaturated. Since the stoichiometry suggests that for every Mg2+, two OH- ions are needed, the concentration of OH- would be 0.2M when precipitation starts. The discussion highlights confusion about the role of Ksp and the source of OH- in the precipitation process. Understanding these concepts is essential for correctly calculating the pH at which Mg(OH)2 begins to precipitate.
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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?
 
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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 :smile:

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

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