Silver chloride (AgCl) has a fixed Ksp at a given temperature. When sodium chloride (NaCl) is dissolved in the same solution, what happens to the amount of AgCl that dissolves?
AMore AgCl dissolves because NaCl increases the overall ionic strength of the solution
BLess AgCl dissolves because the added Cl⁻ shifts the dissolution equilibrium toward undissolved solid
CThe same amount dissolves because the Ksp value is unchanged
DMore AgCl dissolves because Na⁺ ions pair with Cl⁻, freeing up capacity for AgCl to dissolve
Ksp is fixed, so with [Cl⁻] already elevated by NaCl, the product [Ag⁺][Cl⁻] would exceed Ksp if AgCl dissolved to the same extent as in pure water. The equilibrium shifts left, reducing [Ag⁺] and therefore reducing AgCl solubility. Option C is the classic mistake: Ksp doesn't change, but solubility does — the pre-existing Cl⁻ uses up part of the ion-product 'budget.'
Question 2 Multiple Choice
In qualitative analysis, adding concentrated HCl to a solution is used to precipitate Ag⁺ as AgCl more completely than plain water would allow. Which principle best explains why HCl is more effective?
AHCl lowers the pH, which destabilizes the Ag⁺ ion in solution
BThe common ion effect: excess Cl⁻ from HCl shifts the AgCl dissolution equilibrium toward the solid
CHCl increases the Ksp of AgCl by changing the temperature of the solution
DHCl reacts directly with Ag⁺ to form a covalent AgCl complex
HCl fully dissociates, flooding the solution with Cl⁻. This common ion (shared with AgCl) shifts the equilibrium AgCl(s) ⇌ Ag⁺ + Cl⁻ to the left, driving more AgCl to precipitate. The pH effect of HCl is irrelevant here; it is purely an application of Le Chatelier's principle to the dissolution equilibrium.
Question 3 True / False
The common ion effect reduces solubility mainly when the added salt shares the cation with the sparingly soluble compound — a shared anion has no effect.
TTrue
FFalse
Answer: False
The common ion effect works when EITHER the cation OR the anion is shared. Adding NaCl to AgCl(s) solution suppresses solubility through the common Cl⁻ anion; adding AgNO₃ would suppress it through the common Ag⁺ cation. Any ion that appears in the Ksp expression counts — the constraint is just that Ksp = [Ag⁺][Cl⁻] must not be exceeded.
Question 4 True / False
Adding NaCl to a saturated AgCl solution causes the concentration of Ag⁺ to decrease, even though no Ag⁺ is added or removed directly.
TTrue
FFalse
Answer: True
Because Ksp = [Ag⁺][Cl⁻] is fixed, increasing [Cl⁻] via NaCl forces [Ag⁺] to decrease proportionally. AgCl precipitates until the ion product falls back to Ksp at a lower [Ag⁺]. This is Le Chatelier's principle in action: adding a product (Cl⁻) drives the equilibrium toward the reactant (solid AgCl), reducing the concentration of the other product (Ag⁺).
Question 5 Short Answer
Explain why the solubility of a sparingly soluble salt decreases when a soluble salt sharing one of its ions is added, even though the Ksp value itself does not change.
Think about your answer, then reveal below.
Model answer: Ksp sets the maximum value of the ion-product at equilibrium. When a common ion is added from a separate soluble salt, the concentration of that ion rises immediately, pushing the ion product above Ksp. To restore equilibrium, the sparingly soluble salt precipitates — shifting the equilibrium left — until the ion product returns to Ksp. At this new equilibrium, the concentration of the other ion (not added externally) is lower than it was in pure water. Solubility is defined as how much of the sparingly soluble salt dissolves, and since less of it is in solution, its solubility has decreased — all while Ksp remains constant.
The key is that Ksp constrains the product of ion concentrations, not either concentration individually. A fixed product means the two concentrations trade off: raise one and the other must fall. The common ion effect is simply the practical consequence of this constraint when an outside source contributes one of the ions.