The common ion effect MCAT describes how adding a shared ion shifts weak acid or base equilibria, directly testing your understanding of Le Châtelier’s principle on the exam. This concept is fundamental for multiple choice questions and passage based problems in general chemistry and biochemistry sections.
Mastery of this topic helps you predict solubility changes, buffer behavior, and percent ionization under new conditions, which are high yield topics for the MCAT.
Common Ion Effect on Solubility
How Shared Ions Reduce Dissolution
When a salt dissolves in a solution that already contains one of its ions, the increased ion concentration shifts the dissolution equilibrium toward the solid, lowering solubility. This directional shift is a direct application of Le Châtelier’s principle and appears frequently in solubility and buffer questions on the MCAT.
| Compound | Solubility in Pure Water (mol/L) | Solubility with Common Ion (mol/L) | Primary Shift Direction |
|---|---|---|---|
| AgCl | 1.3 × 10⁻⁵ | ≈10⁻⁹ (in 0.1 M Cl⁻) | Left |
| CaF₂ | 2.1 × 10⁻⁴ | ≈2 × 10⁻⁶ (in 0.01 M F⁻) | Left |
| Mg(OH)₂ | 1.1 × 10⁻⁴ | ≈4 × 10⁻⁸ (in 0.05 M OH⁻) | Left |
| PbI₂ | 1.4 × 10⁻³ | ≈10⁻⁵ (in 0.1 M I⁻) | Left |
Common Ion Effect on Weak Acids
Buffers and Percent Ionization Reduction
Adding the conjugate base of a weak acid to a solution decreases percent ionization, a core mechanism behind buffer action. The shared ion suppresses dissociation, stabilizing pH and altering equilibrium concentrations in ways tested on the MCAT.
Common Ion Effect on Weak Bases
Suppressing Equilibrium in Amine Systems
Introducing a common cation from a weak base reduces hydroxide production and shifts the base equilibrium left. This principle explains why ammonia in the presence of ammonium salts exhibits minimal further protonation, a pattern common in physiology and pharmacology passages.
Quantitative Calculations and Equations
Using Equilibrium Expressions
Solving common ion problems requires applying Ka, Kb, and Ksp expressions while accounting for initial concentrations of the shared ion. You must often set up an ICE table, simplify by assuming small x, and verify that the approximation holds, which mirrors the stepwise logic expected on the test.
Key Strategies for MCAT Success
- Identify the shared ion first and note its initial concentration.
- Write the relevant equilibrium expression and plug in known values.
- Use an ICE table to organize changes and approximate when appropriate.
- Predict qualitative shifts in solubility, pH, and percent ionization.
- Practice both pure solvent and common ion scenarios to build intuition.
FAQ
Reader questions
How does the common ion effect change solubility compared to pure water?
Solubility decreases because the added ion shifts the dissolution equilibrium toward the solid, lowering the concentration of dissolved ions.
Why does adding acetate reduce percent ionization of acetic acid?
Acetate is the conjugate base; its increase pushes the weak acid equilibrium left, suppressing dissociation and reducing percent ionization.
What role does the common ion play in buffer systems?
The common ion stabilizes pH by minimizing changes in hydrogen ion concentration when small amounts of acid or base is added.
How do you set up an ICE table when a common ion is present?
Include the initial concentration of the common ion, define changes for the weak species, and solve for equilibrium concentrations using the appropriate equilibrium constant.