AP Chemistry Flashcards: Common-Ion Effect
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
What is the overall impact of a common ion on the molar solubility of a sparingly soluble salt?
The presence of a common ion decreases the molar solubility of a sparingly soluble salt compared to its solubility in pure water.
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What is the overall impact of a common ion on the molar solubility of a sparingly soluble salt?
The presence of a common ion decreases the molar solubility of a sparingly soluble salt compared to its solubility in pure water.
In the context of solubility, what is a 'common ion'?
A common ion is an ion that is already present in a solution and is also a component of a salt being dissolved into that same solution.
Does adding a common ion to a solution change the value of Ksp for the salt?
No, the Ksp is an equilibrium constant that only changes with temperature; the common-ion effect alters the molar solubility, not the value of Ksp itself.
How is the solubility product constant (Ksp) used to calculate solubility in the presence of a common ion?
The Ksp expression is used along with the initial concentration of the common ion to solve for the molar solubility of the salt, which represents the equilibrium concentration of the other ion.
What is the common-ion effect?
The common-ion effect is the reduction in the solubility of a salt when it is dissolved into a solution that already contains one of the ions present in that salt.
When calculating the solubility of a salt in a solution with a common ion, what is the first step?
The first step is to identify the common ion and its initial concentration already present in the solution before the salt begins to dissolve.
What two approaches can be used to analyze the common-ion effect?
The common-ion effect can be understood qualitatively by applying Le Châtelier’s principle or analyzed quantitatively by performing calculations with the Ksp value.
If solid silver chromate (Ag₂CrO₄) is added to a solution of 0.1 M silver nitrate (AgNO₃), what is the common ion?
The common ion is the silver ion (Ag⁺), as it is produced by the dissolution of Ag₂CrO₄ and is also already present in the AgNO₃ solution.
How does Le Châtelier’s principle explain the common-ion effect?
The presence of a common ion increases the concentration of a product in the dissolution equilibrium, causing the equilibrium to shift left towards the solid reactant, thus reducing the salt's solubility.
Will lead(II) chloride (PbCl₂) be more soluble in pure water or in a 0.5 M solution of sodium chloride (NaCl)?
PbCl₂ will be less soluble in the 0.5 M NaCl solution because of the common-ion effect caused by the presence of chloride ions (Cl⁻).