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Ch.17 - Additional Aspects of Aqueous Equilibria
All textbooksBrown 14th EditionCh.17 - Additional Aspects of Aqueous EquilibriaProblem 6
Chapter 17, Problem 6

Consider a beaker containing a saturated solution of PbI2 in equilibrium with undissolved PbI2(s). Now solid KI is added to this solution. (a) Will the amount of solid PbI2 at the bottom of the beaker increase, decrease, or remain the same? (b) Will the concentration of Pb2+ ions in solution increase or decrease? (c) Will the concentration of I- ions in solution increase or decrease?

Verified step by step guidance
1
Identify the initial equilibrium condition: The saturated solution of PbI2 is in equilibrium with its solid form, represented by the equation: PbI2(s) ⇌ Pb²⁺(aq) + 2I⁻(aq).
Understand the effect of adding KI: Adding solid KI to the solution increases the concentration of I⁻ ions because KI dissociates into K⁺ and I⁻ ions in solution.
Apply Le Chatelier's Principle: According to Le Chatelier's Principle, the system will respond to the increase in I⁻ concentration by shifting the equilibrium to the left to reduce the disturbance, meaning more PbI2 will precipitate out of the solution.
Analyze the effect on Pb²⁺ concentration: As the equilibrium shifts to the left, the concentration of Pb²⁺ ions in the solution will decrease because Pb²⁺ ions are being used to form more solid PbI2.
Conclude the effect on I⁻ concentration: Although the equilibrium shift reduces the I⁻ concentration slightly, the overall concentration of I⁻ ions in the solution will increase due to the continuous addition of I⁻ from the dissociation of KI.

Key Concepts

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

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust to counteract the change and restore a new equilibrium. In the context of the PbI2 solution, adding KI introduces more I- ions, which affects the equilibrium position of the dissolution of PbI2.
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Le Chatelier's Principle

Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is an equilibrium constant that applies to the solubility of sparingly soluble ionic compounds. For PbI2, the Ksp expression involves the concentrations of Pb2+ and I- ions. When the concentration of I- ions increases due to the addition of KI, it can shift the equilibrium, affecting the solubility of PbI2.
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Common Ion Effect

The common ion effect refers to the decrease in solubility of an ionic compound when a common ion is added to the solution. In this case, adding KI increases the concentration of I- ions, which will shift the equilibrium of the PbI2 dissolution reaction, leading to a decrease in the solubility of PbI2 and affecting the amounts of Pb2+ and I- ions in solution.
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Related Practice
Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (a) Which of the three drawings, (1), (2), or (3), represents the buffer after the addition of a strong acid? [Section 17.2]

Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (c) Which of the three represents a situation that cannot arise from the addition of either an acid or a base? [Section 17.2]

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Textbook Question

The following figure represents solutions at various stages of the titration of a weak acid, HA, with NaOH. (The Na+ ions and water molecules have been omitted for clarity.) To which of the following regions of the titration curve does each drawing correspond: (b) after addition of NaOH but before the equivalence point? [Section 17.3]

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Textbook Question

Match the following descriptions of titration curves with the diagrams: (a) strong acid added to strong base. [Section 17.3]

Textbook Question

Match the following descriptions of titration curves with the diagrams: (d) strong base added to polyprotic acid. [Section 17.3]