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

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SUMMARY

The discussion focuses on calculating the pH at which Magnesium Hydroxide (Mg(OH)2) begins to precipitate from a 0.1M Mg2+ solution, given a solubility product constant (Ksp) of 10^-11. The key insight is that the concentration of hydroxide ions (OH-) required for precipitation is derived from the stoichiometry of the reaction, which indicates that for every Mg2+ ion, two OH- ions are needed, leading to a concentration of 0.2M OH-. The misunderstanding arises from the role of Ksp in determining the precipitation point, emphasizing the importance of recognizing the stoichiometric relationships in the reaction.

PREREQUISITES
  • Understanding of solubility product constants (Ksp)
  • Knowledge of stoichiometry in chemical reactions
  • Familiarity with acid-base equilibria
  • Basic principles of precipitation reactions
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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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