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Weak Acid and Base Equilibria

A weak acid (or base) only partially ionizes, reaching an equilibrium between the molecule and its ions. The extent is captured by the acid ionization constant Ka (or Kb for a base): a larger Ka means a stronger weak acid, one that ionizes more.

UNIT 8 TOPIC 8.3 • WEAK ACID AND BASE EQUILIBRIA Ka AND Kb Weak acids and bases only partially ionize — most particles remain as reactants at equilibrium. PARTICLE VIEWS AT EQUILIBRIUM Weak acid (HA) HA HA HA HA HA H₃O⁺ A⁻ HA A⁻ HA H₃O⁺ HA HA + H₂O ⇌ H₃O⁺ + A⁻ Weak base (B) B B B B B BH⁺ B OH⁻ BH⁺ B OH⁻ B B + H₂O ⇌ BH⁺ + OH⁻ WEAK ACID EQUILIBRIUM HA + H₂O ⇌ H₃O⁺ + A⁻ Ka = [H₃O⁺][A⁻] / [HA] WEAK BASE EQUILIBRIUM B + H₂O ⇌ BH⁺ + OH⁻ Kb = [BH⁺][OH⁻] / [B] LOGARITHMIC FORMS pKa = −log Ka pKb = −log Kb A smaller pKa (larger Ka) means a stronger acid. CONJUGATE PAIR RELATION CED ANCHOR For a conjugate acid–base pair (HA / A⁻ or B / BH⁺): Ka × Kb = Kw = 1.0×10⁻¹⁴ (at 25 °C) SMALL Ka / Kb Reactants are favored at equilibrium. Low percent ionization. DILUTION Percent ionization increases as the concentration decreases. AP Chemistry · Unit 8 · Acids and Bases
A weak acid only partially ionizes, sitting at equilibrium with its ions. The acid ionization constant Ka measures how far it ionizes: a larger Ka means a stronger weak acid. (Kb plays the same role for weak bases.)
Ka Explorer · Open the sandbox →

The traps treat a weak acid as if it fully ionized (it does not — that is what makes it weak) and misjudge relative strength (compare Ka values; larger is stronger). A weak acid is an equilibrium problem, not a complete reaction.

The work

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Lesson
Weak Acid/Base Equilibria ›

Weak acids and bases partially ionize, with Ka (or Kb) measuring strength — larger is stronger. The lesson treats them as equilibria, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 8.3 misconceptions: a weak acid treated as fully ionized, and weak-acid strength misjudged from Ka.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
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