AP Chemistry Unit 8 Cheat Sheet

The basics

What it covers: Proton transfer — Bronsted-Lowry acids/bases, pH/pOH, weak acid/base equilibria, molecular structure and acid strength, titrations, and buffers.

Exam weight: About 11–15% of the AP Chemistry exam — the second-highest weighted unit.

The big question: How do molecular structure and equilibrium together determine the strength and behavior of acids and bases in solution?

Big Ideas covered: Structure & Properties (SAP), Chemical Effects (CE).

The key terms you must know

• Bronsted-Lowry acid / base — proton donor / proton acceptor.
• Conjugate acid-base pair — two species differing by exactly one H⁺.
• Ka / Kb — equilibrium constants for weak acid/base dissociation; Ka × Kb = Kw for a conjugate pair.
• pH = −log[H₃O⁺] / pOH = −log[OH⁻] — pH + pOH = 14 at 25°C.
• Equivalence point / half-equivalence point — key reference points on a titration curve.
• pKa = −log(Ka) — smaller pKa means a stronger acid.
• Buffer — weak acid + conjugate base mixture that resists pH change.
• Henderson-Hasselbalch equation — pH = pKa + log([conjugate base]/[acid]).

Key themes to remember

• Strong vs. weak is a calculation difference, not a different chemical concept. Strong = direct calculation; weak = equilibrium calculation.
• Molecular structure predicts strength before you ever calculate a number. More electronegative pull (more O atoms, higher electronegativity) means a stronger acid.
• A titration curve is a story about equilibrium shifting in real time. Every point on the curve corresponds to a specific Q vs. K relationship.
• A buffer's best operating range is near its pKa. Buffer capacity is highest when [acid] ≈ [conjugate base], i.e., near the half-equivalence point.
• This unit is equilibrium (Unit 7) with a more specific vocabulary. Every Ka/Kb problem is really just a Unit 7 ICE table problem in disguise.

Common exam traps

• Don't use the strong-acid shortcut on a weak acid. Weak acids require an ICE table with Ka — you cannot just divide concentration directly into pH.
• The equivalence point of a weak acid/strong base titration is NOT pH 7. It's basic, because the conjugate base of the weak acid hydrolyzes water.
• pH = pKa happens at the half-equivalence point, not the equivalence point. Don't confuse these two landmarks on a titration curve.
• A buffer doesn't prevent pH change entirely — it resists it. Add enough acid or base and the buffer will eventually be overwhelmed.
• Don't forget that water itself contributes H₃O⁺ and OH⁻. This usually doesn't matter except for extremely dilute solutions or very weak acids/bases.