Chemistry • Year 12 • Module 6 • Lesson 12
Ka, pKa & Comparing Acid Strengths
Lock in the core vocabulary, the Ka–pKa inverse relationship, and how to rank acids from data tables.
1. Term–definition match
The twelve definitions below are shuffled. In the right-hand column write the matching term from this list: Ka (acid dissociation constant), pKa, acid strength, degree of ionisation, conjugate base, conjugate acid, Kb, Ka × Kb = Kw, polyprotic acid, Ka1, levelling effect, intrinsic molecular property. 12 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The equilibrium constant for the ionisation of a weak acid in water; Ka = [H⁺][A⁻] / [HA]. | |
| 1.2 | The negative logarithm (base 10) of Ka; a more convenient scale for comparing weak acids. | |
| 1.3 | How readily an acid donates a proton to water — determined by Ka, not by pH. | |
| 1.4 | The fraction of acid molecules that have actually ionised in a given solution, expressed as a percentage. | |
| 1.5 | The species formed when an acid donates a proton; it is the acid minus one H⁺. | |
| 1.6 | The species formed when a base accepts a proton; it is the base plus one H⁺. | |
| 1.7 | The base dissociation constant for a conjugate base; measures its tendency to accept a proton from water. | |
| 1.8 | The relationship linking an acid's dissociation constant and its conjugate base's dissociation constant at 25 °C; the product equals 1.0 × 10⁻¹⁴. | |
| 1.9 | An acid that can donate more than one proton in successive ionisation steps, each with its own dissociation constant. | |
| 1.10 | The first-step dissociation constant of a polyprotic acid; always the largest and the only one used for routine pH calculations. | |
| 1.11 | The phenomenon whereby all strong acids appear equally strong in water because water fully ionises them — true differences are only visible in non-aqueous solvents. | |
| 1.12 | A quantity that depends only on the identity of the acid and temperature, not on concentration — Ka and pKa are examples. |
2. True or false — with correction
Circle T or F for each statement. If false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction each)
2.1 A larger Ka value means the acid is stronger because more molecules have ionised at equilibrium. T / F
2.2 A larger pKa value indicates a stronger acid because pKa and Ka increase together. T / F
2.3 pH can be used to rank acid strength even when the acids are at different concentrations. T / F
2.4 The relationship Ka × Kb = Kw applies to any acid–base pair, not just a conjugate pair. T / F
2.5 For H₂CO₃, Ka1 is roughly 10,000 times larger than Ka2, so only Ka1 is needed to calculate solution pH. T / F
3. Complete the passage — Ka, pKa and acid strength
Fill each blank using one word or expression from the word bank below. Each term is used once. 8 marks
Ka is an molecular property — it depends only on the identity of the acid and temperature, never on . This means Ka is the correct quantity for comparing acid strengths. The acid with the Ka is the acid in a set. Because pKa = −log(Ka), a Ka gives a pKa. A useful mnemonic is that pKa is like a score — lower is better (stronger acid). For any conjugate acid–base pair, Ka × Kb = , which means the the acid, the stronger its conjugate base.
4. Short-answer recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 Why is Ka described as an intrinsic molecular property of an acid, whereas pH is not?
4.2 Using the formula pKa = −log(Ka), explain why larger Ka corresponds to smaller pKa.
4.3 State the formula linking Ka and Kb for a conjugate pair, and explain what it tells you about the strength of a conjugate base relative to the strength of its parent acid.
4.4 Explain, using an electrostatic argument, why Ka2 of H₃PO₄ is approximately 10⁵ times smaller than Ka1.
4.5 Write the Ka expression for the first ionisation of carbonic acid (H₂CO₃) in water.
5. Rank and classify the five acids
Use the Ka data table below to complete columns C–E. The first row is done as a model. 8 marks
| A — Acid | B — Ka | C — pKa (to 2 d.p.) |
D — Strength label (very weak / weak / moderately weak) |
E — Rank (1 = strongest) |
|---|---|---|---|---|
| HF | 6.8 × 10⁻⁴ | 3.17 | Moderately weak | 1 |
| CH₃COOH | 1.8 × 10⁻⁵ | |||
| H₂CO₃ | 4.3 × 10⁻⁷ | |||
| HCN | 6.2 × 10⁻¹⁰ | |||
| H₂O | 1 × 10⁻¹⁶ |
5.1 Which conjugate base in the table above is the strongest base? Justify your answer using Ka × Kb = Kw.
Q1 — Term–definition match
1.1 Ka (acid dissociation constant) • 1.2 pKa • 1.3 acid strength • 1.4 degree of ionisation • 1.5 conjugate base • 1.6 conjugate acid • 1.7 Kb • 1.8 Ka × Kb = Kw • 1.9 polyprotic acid • 1.10 Ka1 • 1.11 levelling effect • 1.12 intrinsic molecular property.
Q2 — True / false
2.1 True.
2.2 False. Larger pKa means smaller Ka, which means a weaker acid. pKa and acid strength move in opposite directions.
2.3 False. pH depends on both Ka and concentration; two acids at different concentrations can have the same pH even if their Ka values differ. Only Ka is an intrinsic measure of acid strength and can be used to rank acids regardless of concentration.
2.4 False. Ka × Kb = Kw applies only to a conjugate pair — Ka(HA) × Kb(A⁻) = Kw, where A⁻ is the conjugate base formed when HA donates a proton. It does not apply to unrelated acid–base pairs.
2.5 True. Ka1/Ka2 = (4.3 × 10⁻⁷)/(4.7 × 10⁻¹¹) ≈ 9,100 ≈ 10⁴. The second ionisation contributes less than 0.01% of total [H⁺] and is ignored in routine calculations.
Q3 — Cloze
intrinsic • concentration • larger • strongest • larger • smaller • golf • Kw • weaker
Q4 — Short-answer recall
4.1 Ka depends only on the identity of the acid and temperature; it does not change when concentration changes. pH = −log[H⁺] reflects both Ka and concentration, so two solutions can have the same pH but different Ka values.
4.2 pKa = −log(Ka). A larger Ka means a more positive number inside the logarithm, so the logarithm itself is larger, and the negative of that larger number is smaller. Hence larger Ka → smaller pKa.
4.3 Ka(HA) × Kb(A⁻) = Kw = 1.0 × 10⁻¹⁴ at 25 °C. This means the stronger the acid (larger Ka), the smaller Kb of its conjugate base — conjugate bases of strong acids are very weak bases, and conjugate bases of weak acids are relatively stronger bases.
4.4 Removing the second proton (H⁺) from H₂PO₄⁻ requires overcoming greater electrostatic attraction because H₂PO₄⁻ carries a negative charge. The positive H⁺ is attracted to the negatively charged ion and is much harder to remove than from the neutral H₃PO₄ molecule. Each successive negative charge on the ion makes subsequent proton removal progressively harder (smaller Ka).
4.5 H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq) • Ka1 = [H⁺][HCO₃⁻] / [H₂CO₃] = 4.3 × 10⁻⁷
Q5 — Ranking table
CH₃COOH: pKa = 4.74 • Weak • Rank 2 | H₂CO₃: pKa = 6.37 • Very weak • Rank 3 | HCN: pKa = 9.21 • Very weak • Rank 4 | H₂O: pKa = 16.0 • Extremely weak • Rank 5.
5.1 The conjugate base of H₂O (OH⁻) is the strongest base. H₂O has the smallest Ka (1 × 10⁻¹⁶), so by Kb = Kw/Ka, Kb(OH⁻) = 1.0 × 10⁻¹⁴ / 1 × 10⁻¹⁶ = 100 — indicating OH⁻ is a very strong base, consistent with NaOH being a strong base.