Answer:
The correct answer is "Set the pH to acidic conditions (pH 3) and Fe(OH)3 will precipitate but not Mg(OH)2".
Explanation:
The value of equilibrium constant (Ksp) determines the degree at which a component solubilizes in water. The higher the Ksp, the more easily a substance is solubilized in water. In this case, the Ksp is determined by the concentration of hydroxides and Mg(OH)2 has a much higher Ksp value than Fe(OH)3. Therefore, the best way to precipitate one ion but not the other will be to put the ions at a low hydroxide concentration, such at an acidic pH. In this condition Fe(OH)3 will precipitate but not Mg(OH)2, because the latter is much more soluble and will be soluble even at a low hydroxide concentration. I attached the missing Ksp values and the missing options for this question.
