HSC Chemistry — Module 6
Buffers — Flashcards & Quiz
Buffer solutions resist pH change when small amounts of acid or base are added, and HSC Chemistry Module 6 uses them to test your conjugate-pair fluency. A buffer is a weak acid + its conjugate base (or vice versa). You need to apply the Henderson-Hasselbalch equation, identify effective buffer ranges, and link the concept to biological systems such as the bicarbonate buffer in blood plasma — a favourite exam context.
Key Points
- A buffer is a solution that resists pH change when small amounts of strong acid or base are added.
- Composition: weak acid + its conjugate base (e.g. CH₃COOH / CH₃COO⁻) OR weak base + its conjugate acid (e.g. NH₃ / NH₄⁺).
- Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). Effective buffer range is pKa ± 1.
- Mechanism: added acid reacts with conjugate base; added base reacts with weak acid — neither directly changes [H⁺] dramatically.
- Buffer capacity = the amount of acid or base the buffer can neutralise before pH shifts significantly; depends on total [HA] + [A⁻].
- Biological example: bicarbonate buffer in blood (H₂CO₃ / HCO₃⁻) maintains pH ~7.4 — a favourite HSC exam context.
Common Mistakes to Avoid
- Thinking any weak acid/base is a buffer — you need BOTH the weak acid AND its conjugate base.
- Using pure water as a buffer — water has no resistance to pH change.
- Applying Henderson-Hasselbalch outside the effective buffer range (pKa ± 1).
- Confusing buffer capacity with buffer range — they're different concepts.
- Missing the biological importance of the bicarbonate buffer in blood (the most common HSC exam example).
Exam Strategy
HSC Module 6 buffer questions ask you to (1) identify whether a solution is a buffer, (2) calculate pH using Henderson-Hasselbalch, or (3) explain buffering in a biological system. Method: confirm the presence of a weak acid AND conjugate base, apply pH = pKa + log([A⁻]/[HA]), reference the bicarbonate buffer for biology applications.
Sample Flashcards
Q1: What is a buffer solution and how does it work?
A buffer resists pH changes when small amounts of acid or base are added. It consists of a weak acid and its conjugate base (or weak base and conjugate acid). If H⁺ is added, the conjugate base neutralises it. If OH⁻ is added, the weak acid neutralises it.
Q2: Give an example of a biologically important buffer.
Blood uses the carbonate buffer system: H₂CO₃ ⇌ H⁺ + HCO₃⁻. It maintains blood pH at ~7.4. If H⁺ increases: HCO₃⁻ (base) neutralises it. If H⁺ decreases: H₂CO₃ (acid) releases more H⁺. This prevents dangerous pH swings that would denature enzymes.
Sample Quiz Questions
Q1: A buffer solution is made from a strong acid and its conjugate base.
Answer: FALSE
A buffer is made from a WEAK acid and its conjugate base (or a weak base and its conjugate acid). Strong acids fully dissociate and cannot form buffers.
Q2: The blood bicarbonate buffer system maintains blood pH at approximately 7.4.
Answer: TRUE
The H₂CO₃/HCO₃⁻ buffer system in blood absorbs excess H⁺ or OH⁻ to maintain the narrow pH range of 7.35-7.45 essential for enzyme function.
Revision Tip
Henderson-Hasselbalch calculations are formulaic — drill a Revizi deck of 10 buffer problems with different acid/base ratios.
Related Concepts
Last updated: March 2026 · 2 flashcards · 2 quiz questions