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Ch.18 - Chemistry of the Environment
All textbooksBrown 14th EditionCh.18 - Chemistry of the EnvironmentProblem 75
Chapter 18, Problem 75

Explain why Mg(OH)2 precipitates when CO32- ion is added to a solution containing Mg2+.

Verified step by step guidance
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Understand the solubility product concept: The solubility product constant (Ksp) is used to describe the equilibrium between a solid and its ions in a solution. For magnesium hydroxide, Mg(OH)2, the Ksp expression is: Ksp = [Mg2+][OH-]2.
Consider the common ion effect: When CO32- is added to the solution, it can react with water to form OH- ions through the equilibrium: CO32- + H2O ⇌ HCO3- + OH-. This increases the concentration of OH- ions in the solution.
Apply Le Chatelier's Principle: The increase in OH- concentration shifts the equilibrium of the Mg(OH)2 dissolution reaction to favor the formation of the solid precipitate, according to Le Chatelier's Principle.
Calculate the ion product: The ion product, Q, is calculated using the concentrations of Mg2+ and OH- ions. If Q exceeds the Ksp of Mg(OH)2, precipitation occurs.
Conclude the precipitation condition: Since the addition of CO32- increases OH- concentration, the ion product Q becomes greater than the Ksp, leading to the precipitation of Mg(OH)2.

Key Concepts

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

Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is a numerical value that represents the equilibrium between a solid and its ions in a saturated solution. For a compound like Mg(OH)2, Ksp indicates the maximum concentration of Mg2+ and OH- ions that can exist in solution before precipitation occurs. When the product of the ion concentrations exceeds Ksp, the compound will precipitate out of solution.
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Solubility Product Constant

Common Ion Effect

The common ion effect refers to the decrease in solubility of a salt when a common ion is added to the solution. In this case, adding CO3^2- introduces a common ion that can interact with Mg2+ ions, leading to the formation of insoluble compounds. This effect shifts the equilibrium, promoting the precipitation of Mg(OH)2 as the system attempts to reduce the concentration of the common ions.
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Precipitation Reactions

Precipitation reactions occur when two soluble salts react to form an insoluble product, or precipitate. In the context of the question, the addition of CO3^2- ions to a solution containing Mg2+ leads to the formation of Mg(OH)2, which is insoluble in water. This reaction is driven by the low solubility of the resulting compound, causing it to separate from the solution as a solid.
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Related Practice
Textbook Question

The solar power striking Earth every day averages 168 watts per square meter. The highest ever recorded electrical power usage in New York City was 13,200 MW. A record established in July of 2013. Considering that present technology for solar energy conversion is about 10% efficient, from how many square meters of land must sunlight be collected in order to provide this peak power? (For compar- ison, the total area of New York City is 830 km2.)

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Textbook Question

Write balanced chemical equations for each of the following reactions: (a) The nitric oxide molecule undergoes photodissociation in the upper atmosphere. (b) The nitric oxide molecule undergoes photoionization in the upper atmosphere. (c) Nitric oxide undergoes oxidation by ozone in the stratosphere.

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Textbook Question

Write balanced chemical equations for each of the following reactions: (d) Nitrogen dioxide dissolves in water to form nitric acid and nitric oxide.

Textbook Question

(b) Will Mg(OH)2 precipitate when 4.0 g of Na2CO3 is added to 1.00 L of a solution containing 125 ppm of Mg2+?

Textbook Question

(a) The EPA threshold for acceptable levels of lead ions in water is 615 ppb. What is the molarity of an aqueous solution with a concentration of 15 ppb?

Textbook Question

(b) Concentrations of lead in the bloodstream are often quoted in units of μg/dL. Averaged over the entire country, the mean concentration of lead in the blood was measured to be 1.6 μg/dL in 2008. Express this concentration in ppb.