Topic 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)
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 essentially fully ionised in dilute aqueous solution; a weak acid establishes an equilibrium with only a fraction ionised. For a weak monoprotic acid, HA(aq) ⇌ H+(aq) + A-(aq). A strong base such as dissolved NaOH provides hydroxide essentially 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 analytical concentration, a weak monoprotic acid usually gives a lower hydrogen-ion concentration than a strong monoprotic acid. Dilution can increase the fraction of weak acid ionised even while the absolute hydrogen-ion concentration decreases. Degree of ionisation and [H+] must not be treated as the same quantity.
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-. 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-. 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. |
At 298 K, Kw is approximately 1.00 × 10-14 mol2 dm-6, so pH + pOH = 14.00 under the usual concentration convention. Kw changes with temperature. A neutral solution always has [H+] = [OH-], but neutral pH is exactly 7.00 only when pKw = 14.00.
For a conjugate pair HA/A-, Ka(HA)Kb(A-) = Kw, hence pKa + pKb = pKw. The species must be a conjugate pair. Multiplying the acid constant of ethanoic acid by the base constant of ammonia does not give Kw.
Worked example
Find the strength of a conjugate base
At 298 K, ethanoic acid has Ka = 1.80 × 10-5. Find Kb for ethanoate.
- Ethanoate is the conjugate base of ethanoic acid.
- Kb = Kw/Ka = (1.00 × 10-14)/(1.80 × 10-5).
Kb = 5.56 × 10-10. Ethanoate is a weak base and its aqueous salt solution is alkaline.