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Ch.18 - Aqueous Ionic Equilibrium
Tro - Chemistry: A Molecular Approach 6th Edition
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook
All textbooksTro 6th EditionCh.18 - Aqueous Ionic EquilibriumProblem 109
Chapter 18, Problem 109

Predict whether a precipitate will form if you mix 75.0 mL of a NaOH solution with pOH = 2.58 with 125.0 mL of a 0.018 M MgCl2 solution. Identify the precipitate, if any.

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1
Calculate the concentration of OH⁻ ions in the NaOH solution using the pOH value: \[ \text{[OH}^-\text{]} = 10^{-\text{pOH}} \].
Determine the moles of OH⁻ ions in the 75.0 mL NaOH solution by multiplying the concentration of OH⁻ by the volume in liters.
Calculate the moles of Mg²⁺ ions in the 125.0 mL MgCl₂ solution using the concentration and volume.
Determine the final concentrations of OH⁻ and Mg²⁺ ions after mixing by dividing the moles of each ion by the total volume of the mixed solution.
Use the solubility product constant (Ksp) for Mg(OH)₂ to determine if the product of the ion concentrations \([\text{Mg}^{2+}][\text{OH}^-]^2\) exceeds the Ksp, indicating a precipitate will form.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pOH and pH Relationship

pOH is a measure of the hydroxide ion concentration in a solution, and it is related to pH by the equation pH + pOH = 14. A pOH of 2.58 indicates a high concentration of hydroxide ions, which can influence the solubility of certain metal hydroxides, such as magnesium hydroxide.
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Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is an equilibrium constant that describes the solubility of a sparingly soluble ionic compound. For magnesium hydroxide (Mg(OH)2), the Ksp value helps determine whether a precipitate will form when magnesium ions from MgCl2 react with hydroxide ions from NaOH.
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Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to form an insoluble compound, or precipitate. In this case, mixing NaOH and MgCl2 can lead to the formation of magnesium hydroxide, which is insoluble in water, thus resulting in a precipitate if the product of the ion concentrations exceeds the Ksp.
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