Chemistry • Year 12 • Module 6 • Lesson 13
Buffers: Mechanism, Calculations & Natural Systems
Lock in core buffer vocabulary, the two buffer mechanism equations, and the Henderson-Hasselbalch equation before moving to calculations.
1. Label the buffer comparison diagram
The diagram below shows two beakers — one containing pure water and one containing an acetic acid / acetate buffer — being treated with the same amount of HCl. It also shows the mechanism inside the buffer beaker. Write the missing labels into boxes A–H. Each label is drawn from the lesson's Key Terms and buffer mechanism equations. 8 marks
- A — pH change in Beaker A when HCl added _______________________
- B — species responsible for large pH drop in Beaker A _______________________
- C — species in the buffer that reacts with added H⁺ _______________________
- D — ionic equation for that buffer reaction _______________________
- E — reason the pH change in Beaker B is small _______________________
- F — weak acid component of the acetate buffer _______________________
- G — conjugate base component of the acetate buffer _______________________
- H — name of the equation that relates buffer pH to pKa and concentration ratio _______________________
2. Term–definition match
Ten definitions are shuffled below. Write the matching term from this list in the right-hand column:
buffer solution • conjugate base • Henderson-Hasselbalch equation • buffer capacity • weak acid • pKa • partial neutralisation • acidosis • alkalosis • effective buffer range. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | A solution that resists significant pH change when small amounts of strong acid or base are added, containing a weak acid and its conjugate base in comparable concentrations. | |
| 2.2 | The species formed when a weak acid donates a proton; it reacts with added H⁺ to protect the buffer from pH decrease. | |
| 2.3 | pH = pKa + log([A⁻]/[HA]); the equation relating buffer pH to the ratio of conjugate base to weak acid concentration. | |
| 2.4 | The amount of strong acid or base a buffer can absorb before significant pH change occurs; maximised when [HA] = [A⁻]. | |
| 2.5 | An acid that only partially ionises in water, establishing an equilibrium between the undissociated molecule and its ions. | |
| 2.6 | −log10(Ka); the negative logarithm of the acid dissociation constant; equals buffer pH when [A⁻] = [HA]. | |
| 2.7 | A buffer preparation method in which a measured volume of strong base is added to a weak acid, converting a known fraction of HA to A⁻. | |
| 2.8 | A clinical condition in which blood pH falls below 7.35 due to excess acid load or loss of bicarbonate. | |
| 2.9 | A clinical condition in which blood pH rises above 7.45, often caused by excessive CO₂ expulsion through hyperventilation. | |
| 2.10 | The pH range approximately one unit either side of pKa within which a buffer resists pH change effectively (pKa ± 1). |
3. True or false — with correction
Circle T or F for each statement. If false, write the corrected version on the line below. 10 marks (1 mark T/F + 1 mark correction each)
3.1 A buffer solution must have a pH of 7 to function correctly. T / F
3.2 When a small amount of HCl is added to an acetic acid / sodium acetate buffer, the weak acid (CH₃COOH) reacts with the added H⁺ to prevent pH change. T / F
3.3 Buffer capacity is maximised when [A⁻] = [HA] because both components are present in the highest possible amounts relative to each other at that ratio. T / F
3.4 The blood carbonate buffer H₂CO₃/HCO₃⁻ operates with a [HCO₃⁻]/[H₂CO₃] ratio of approximately 20:1 to maintain blood pH at 7.35–7.45, even though pKa = 6.10. T / F
3.5 Adding equal moles of a weak acid and a strong base always produces a buffer solution. T / F
4. Function recall
Answer each question in 1–2 sentences using precise terms. 8 marks (2 each)
4.1 What is the function of the conjugate base (A⁻) component in a buffer solution when strong acid is added?
4.2 What is the function of the weak acid (HA) component in a buffer solution when strong base is added?
4.3 What is the function of the lungs in maintaining the blood H₂CO₃/HCO₃⁻ buffer system?
4.4 What is the function of the phosphate buffer (H₂PO₄⁻/HPO₄²⁻) inside body cells?
5. Fill-in-the-blank paragraph
Complete the paragraph using the word bank below. Each blank requires exactly one term. 9 marks (1 each)
Word bank: conjugate base • weak acid • Henderson-Hasselbalch • pKa • ratio • resists • capacity • HCO₃⁻ • CO₂
A buffer solution __________ significant pH change when small amounts of strong acid or base are added. It consists of a __________ (HA) and its __________ (A⁻) present simultaneously in comparable concentrations. When H⁺ is added, it reacts with A⁻; when OH⁻ is added, it reacts with HA. The pH of a buffer is calculated using the __________ equation: pH = __________ + log([A⁻]/[HA]). Buffer __________ is greatest when [A⁻] = [HA], because both components can absorb the maximum amount of added acid or base. The effective buffering range spans approximately one pH unit either side of pKa, because outside this range the __________ of A⁻ to HA becomes too extreme. In blood, the primary buffer maintains pH at 7.35–7.45 using H₂CO₃ and __________; the lungs regulate this system by expelling __________.
Q1 — Labelled diagram
A: Large drop (approximately 5 pH units; e.g. pH 7.00 → 2.00). B: No weak base / conjugate base present to consume the H⁺; the H⁺ remains free in solution. C: CH₃COO⁻ (the conjugate base / acetate ion). D: H⁺ + CH₃COO⁻ → CH₃COOH. E: Both [CH₃COO⁻] and [CH₃COOH] are present in large amounts; adding a small amount of H⁺ changes the ratio [A⁻]/[HA] only slightly; the log term in Henderson-Hasselbalch changes very little, so pH changes by only a small amount. F: CH₃COOH (acetic acid / ethanoic acid). G: CH₃COO⁻ (acetate ion). H: Henderson-Hasselbalch equation.
Q2 — Term–definition matches
2.1 buffer solution • 2.2 conjugate base • 2.3 Henderson-Hasselbalch equation • 2.4 buffer capacity • 2.5 weak acid • 2.6 pKa • 2.7 partial neutralisation • 2.8 acidosis • 2.9 alkalosis • 2.10 effective buffer range.
Q3 — True / false with correction
3.1 False. Correction: a buffer does not need a pH of 7; its pH is determined by pKa + log([A⁻]/[HA]). The acetate buffer operates at pH ~4.74 when [A⁻] = [HA] — acidic, yet fully functional.
3.2 False. Correction: it is the conjugate base (CH₃COO⁻) that reacts with added H⁺ via CH₃COO⁻ + H⁺ → CH₃COOH. The weak acid (CH₃COOH) reacts with added OH⁻, not with added H⁺.
3.3 True. When [A⁻] = [HA], neither component is limiting; both can absorb the maximum load of added acid (A⁻ consumed) or base (HA consumed), giving the greatest buffer capacity.
3.4 True. pH = 6.10 + log(20) = 6.10 + 1.30 = 7.40. The ratio 20:1 is confirmed by the Henderson-Hasselbalch equation; the blood operates far from the pKa but maintains capacity because the system is “open” (CO₂ is continuously exhaled).
3.5 False. Correction: if equal moles of weak acid and strong base are mixed, the equivalence point is reached — all HA is converted to A⁻ and no HA remains. No buffer exists. A buffer requires the moles of strong base added to be less than the moles of weak acid present.
Q4.1 — Function of conjugate base
The conjugate base (A⁻) reacts with any added H⁺ via A⁻ + H⁺ → HA, consuming the added acid before it can significantly increase [H⁺] in solution. This is why a buffer resists pH decrease on acid addition.
Q4.2 — Function of weak acid
The weak acid (HA) reacts with any added OH⁻ via HA + OH⁻ → A⁻ + H₂O, consuming the added base before it can significantly decrease [H⁺]. This is why a buffer resists pH increase on base addition.
Q4.3 — Function of the lungs
The lungs expel CO₂ produced from H₂CO₃ decomposition (H₂CO₃ → CO₂ + H₂O), which regenerates buffer capacity by continuously removing the acid component. Increasing breathing rate lowers [H₂CO₃] and raises the [HCO₃⁻]/[H₂CO₃] ratio, raising blood pH; decreasing breathing rate has the opposite effect.
Q4.4 — Function of phosphate buffer in cells
The intracellular phosphate buffer (H₂PO₄⁻/HPO₄²⁻, pKa = 7.21) maintains cytoplasmic pH within the range 6.8–7.4, ensuring metabolic enzymes remain in the correct ionisation state for catalysis. Metabolic reactions that release H⁺ are neutralised by HPO₄²⁻, protecting enzyme activity.
Q5 — Cloze paragraph
In order: resists / weak acid / conjugate base / Henderson-Hasselbalch / pKa / capacity / ratio / HCO₃⁻ / CO₂.