Unit 8: Acids and Bases
Test AP Chemistry Unit 8 — pH, Ka, Kb, buffer solutions, Henderson-Hasselbalch, and titration curves. AP-style MCQ and FRQ practice for acids and bases.
What Unit 8 Covers in AP Chemistry
Acids and bases form one of the most FRQ-rich areas of AP Chemistry. Unit 8 combines equilibrium concepts from Unit 7 with new skills in pH calculation, buffer design, and titration analysis. Strong performance here requires both algebraic fluency and the ability to interpret titration curves graphically.
Core Topics in Unit 8
- Strong and weak acids and bases — Complete vs. partial dissociation; writing dissociation equilibria; identifying conjugate acid-base pairs
- pH, pOH, and the water equilibrium — Calculating pH from concentration for strong acids/bases and weak acids/bases; using Kw at 25 degrees Celsius
- Ka and Kb — Writing Ka and Kb expressions; solving for pH of weak acid or weak base solutions using ICE tables; the relationship Ka times Kb equals Kw
- Buffer solutions — How buffers resist pH change; choosing appropriate acid-conjugate base pairs; calculating pH using the Henderson-Hasselbalch equation
- Henderson-Hasselbalch equation — Applying pH equals pKa plus log([A-]/[HA]) for buffer calculations
- Titration curves — Shape and features of strong acid-strong base and weak acid-strong base titration curves; identifying the equivalence point, half-equivalence point, and buffer region
- Equivalence points — Calculating the pH at the equivalence point using hydrolysis of the conjugate species
AP FRQ Patterns: Buffer Calculations
Buffer FRQs typically give you a weak acid and its conjugate base (or the amount of strong base added to a weak acid) and ask for the pH. A complete solution shows the Henderson-Hasselbalch equation with values substituted correctly. Some FRQs also ask you to explain, at the molecular level, how the buffer resists pH change when acid or base is added.
AP FRQ Patterns: Titration Curve Analysis
Titration curve questions ask you to identify key points on the curve, calculate pH at a specific point, or explain why the equivalence point pH is above or below 7. For weak acid-strong base titrations, the equivalence point pH is above 7 because the conjugate base hydrolyzes. Practice sketching and labelling titration curves for both strong-strong and weak-strong scenarios.
Common Mistakes in Unit 8
- Forgetting to use an ICE table when calculating pH of a weak acid — assuming full dissociation gives an incorrect answer
- Applying Henderson-Hasselbalch outside its valid range (when the ratio of conjugate base to acid is extreme)
- Confusing the equivalence point with the half-equivalence point on titration curves
- Not recognizing that at the half-equivalence point of a weak acid titration, pH equals pKa