8.3 Weak Acid and Base Equilibria

Weak Acids and Bases Only Partially Ionize

Unlike strong acids/bases that fully dissociate, weak acids and bases reach an equilibrium between ionized and un-ionized forms. This is described by Ka (acid dissociation constant) or Kb (base dissociation constant). Smaller Ka or Kb means weaker acid/base and less dissociation. The equilibrium concentrations can be found using a standard RICE table (Reaction, Initial, Change, Equilibrium).

Setting Up the Ka/Kb Equilibrium

For HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA]. Using initial concentration C and letting x = amount ionized, Ka ≈ x²/C when x is small (<5% approximation). Solve for x = [H⁺], then find pH = -log[H⁺]. The same logic applies to weak bases using Kb and [OH⁻], then pOH and pH.

Percent Ionization Depends on Concentration

Percent ionization = (x/C) × 100%, where x is the concentration ionized and C is initial concentration. As a weak acid or base is diluted, percent ionization increases, even though the actual concentration of ions decreases — this follows from Le Chatelier's principle shifting equilibrium toward products.

Key terms

Ka

Acid dissociation constant; equals [H⁺][A⁻]/[HA] at equilibrium for a weak acid.

Kb

Base dissociation constant; describes the equilibrium of a weak base ionizing in water.

Small-x approximation

Assumes x (amount ionized) is negligible compared to initial concentration C, so Ka ≈ x²/C; valid when x < 5% of C.

Percent ionization

(x/C) × 100%, the fraction of a weak acid or base that has ionized at equilibrium.

Effect of dilution on weak acids

Dilution increases percent ionization (shifts equilibrium toward ions) even though actual ion concentration decreases.

Try an AP-style question

0.10 M HA has Ka = 1.0 × 10⁻⁵. What is its pH?

1. 3
2. 5
3. 2
4. 1