Lesson 1 of 6
Acid-base strength and equilibrium constants
Use dissociation extent, conjugate pairs and the ionic product of water.
A-Level 9476 (2026-2027)
I can explain strong/weak acid and base behaviour.Syllabus
Syllabus 9476, 10.1(a). Explain strong/weak acid and base behaviour.
I can define and calculate with acid-base equilibrium quantities.Syllabus
Syllabus 9476, 10.1(b). Define and calculate with acid-base equilibrium quantities.
Strength is the extent of ionisation, not the amount dissolved
A dilute strong acid and a concentrated weak acid are different ideas.
A strong acid is almost fully ionised in dilute solution. A weak acid sets up an equilibrium in which only a small fraction is ionised. For a weak monoprotic acid, HA(aq) ⇌ H+(aq) + A-(aq). In the same way, a strong base such as NaOH releases its hydroxide almost completely. A weak base such as NH3 reacts only partly with water: NH3 + H2O ⇌ NH4+ + OH-.
| Description | Meaning | Example distinction |
|---|---|---|
| Strong or weak | Extent of acid/base ionisation or reaction with water. | HCl is strong; ethanoic acid is weak. |
| Concentrated or dilute | Amount of dissolved substance per volume. | A weak acid can be concentrated. |
| Soluble or sparingly soluble | How much substance can dissolve under the conditions. | A sparingly soluble hydroxide is not automatically a weak base. |
At the same concentration, a weak monoprotic acid gives a lower hydrogen-ion concentration than a strong one, because less of it ionises. Diluting a weak acid can raise the fraction that ionises, even while [H+] itself falls. So the degree of ionisation and [H+] are different quantities. Do not treat them as the same.
Check your understanding
Two solutions are each 0.100 mol dm-3: hydrochloric acid and ethanoic acid. Which statement is correct?
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Magnesium fizzes more slowly at first in ethanoic acid, because its [H+] is lower
HCl ionises fully, so [H+] = 0.100 mol dm-3. Ethanoic acid ionises only partly, so its [H+] is far lower and H+ ions hit the magnesium less often.
Constants connect an acid to its conjugate base
Ka and Kb quantify equilibrium; p values are their negative base-10 logarithms.
| Quantity | Definition and use |
|---|---|
| pH | -log10[H+], using the usual dilute-solution concentration convention. |
| Ka for HA | [H+][A-]/[HA] for HA ⇌ H+ + A-. Units: mol dm-3. A larger Ka means a stronger acid. |
| pKa | -log10 Ka. A smaller pKa means a stronger acid. |
| Kb for B | [BH+][OH-]/[B] for B + H2O ⇌ BH+ + OH-. Units: mol dm-3. A larger Kb means a stronger base. |
| pKb | -log10 Kb. A smaller pKb means a stronger base. |
| Kw | [H+][OH-], the ionic product of water at the stated temperature. Units: mol2 dm-6. |
At 298 K, Kw is about 1.00 × 10-14 mol2 dm-6, so pH + pOH = 14.00. Kw changes with temperature. A neutral solution always has [H+] = [OH-]. Its pH is exactly 7.00 only when pKw = 14.00, that is, at 298 K.
For a conjugate pair HA/A-, Ka(HA) × Kb(A-) = Kw, so pKa + pKb = pKw. This works only for a conjugate pair. For example, Ka of ethanoic acid times Kb of ammonia does not give Kw, because they are not a pair.
Worked exampleFind the strength of a conjugate base
At 298 K, ethanoic acid has Ka = 1.80 × 10-5 mol dm-3. Find Kb for ethanoate.
Try it first. Then show the worked stepsHide the worked steps
- Ethanoate is the conjugate base of ethanoic acid.
- Kb = Kw/Ka = (1.00 × 10-14 mol2 dm-6)/(1.80 × 10-5 mol dm-3).
- Units: mol2 dm-6 divided by mol dm-3 leaves mol dm-3.
Kb = 5.56 × 10-10 mol dm-3. Ethanoate is a weak base and its aqueous salt solution is alkaline.
Check your understanding
Ethanoic acid has pKa 4.76 and methanoic acid has pKa 3.75. Which is the stronger acid?
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Methanoic acid, because a smaller pKa means a larger Ka
pKa = -log Ka, so a smaller pKa means a larger Ka. A larger Ka means the acid ionises more at equilibrium.