Final answer:
The minimum concentration of Mg2+ ions required to precipitate magnesium fluoride from a solution containing 0.0125 M NaF is determined by setting up the equation (concentration of Mg2+)(0.0125)^2 = Ksp and solving for the Mg2+ concentration.
Step-by-step explanation:
To calculate the minimum concentration of Mg2+ ions needed to precipitate magnesium fluoride from a solution containing 0.0125 M NaF, we must first understand the solubility product constant (Ksp) of magnesium fluoride. Given that the Ksp of MgF2 at a certain temperature is 6.89 x 10-9, and assuming that the dissolution of magnesium fluoride is represented by the equation MgF2 (s) ⇌ Mg2+ (aq) + 2F- (aq), the Ksp expression would be [Mg2+][F-]2.
When mixing Mg2+ with F- already present in the solution, the ion product [Mg2+][F-]2 should not exceed the Ksp value for precipitation to be avoided. Since the concentration of fluoride ions is given as 0.0125 M (from NaF), we want to find the concentration of Mg2+ that would cause this ion product to equal the Ksp. This would result in the equation (concentration of Mg2+)(0.0125)2 = 6.89 x 10-9. Solving for the concentration of Mg2+ will yield the minimum concentration required to initiate precipitation.