Example 1
Calculate the pH of 0.0100 mol/dm³ HCl, assuming complete dissociation and negligible water contribution.
- HCl is monoprotic, so [H⁺] = 0.0100 mol/dm³.
- pH = -log₁₀(0.0100) = 2.00.
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H2 acid-base equilibria: pH, weak acids and buffers
Start by identifying the species left after any reaction. A strong acid plus a base first requires stoichiometry; a logarithm cannot replace that step. State the assumptions behind weak-acid approximations and buffer calculations.
Choose the method
For a sufficiently concentrated strong monoprotic acid, [H⁺] is approximately its analytical concentration and pH = -log₁₀[H⁺]. This approximation fails for extremely dilute solutions where water matters.
For a weak monoprotic acid HA of concentration c, Ka = [H⁺][A⁻]/[HA]. If dissociation is small and water is negligible, [H⁺] ≈ √(Ka c). Check that [H⁺]/c is small.
For a buffer containing appreciable HA and A⁻, pH ≈ pKa + log₁₀([A⁻]/[HA]). Added H⁺ reacts with A⁻; added OH⁻ reacts with HA. Capacity is limited by the available amounts.
Worked examples
Example 2
A weak monoprotic acid has Ka = 1.0 × 10⁻⁵ and c = 0.100 mol/dm³. Estimate its pH and check the approximation.
- [H⁺] ≈ √(1.0 × 10⁻⁵ × 0.100) = 1.0 × 10⁻³ mol/dm³.
- pH ≈ 3.00.
- Dissociated fraction ≈ 0.0010/0.100 = 1%. This supports treating [HA] as approximately 0.100 mol/dm³.
Try it yourself
A buffer has pKa = 4.76 and equal concentrations of HA and A⁻. Estimate its pH, and describe what happens when a small amount of acid is added.
Show the worked answer
- The ratio is 1 and log₁₀(1) = 0, so pH ≈ 4.76.
- A⁻ consumes added H⁺ to form HA. The ratio [A⁻]/[HA] decreases slightly, so pH decreases slightly; it does not stay exactly constant.
Common mistakes
- Do not confuse acid strength with concentration.
- A mixture is not automatically a buffer: both a weak acid and its conjugate base must remain in appreciable amounts after reaction.
Sources and review
SEAB 2026 syllabus: H2 Chemistry 9476: acid-base equilibria.
Original Rae practice, prepared with AI assistance. Selected numerical results and their displayed working are automatically checked at publication; this does not verify every explanation. Curriculum references checked on 5 September 2026. No teacher review or SEAB endorsement is claimed.
What the syllabus expects
- Explain qualitatively how strong and weak acids and bases differ, tracing it to how fully they dissociate.
- Define pH, Ka, pKa, Kb, pKb and Kw and use them in calculations, the relationship Kw = KaKb included.
- Compute the hydrogen-ion concentration and pH of strong acids, of strong bases, of weak monoacidic bases and of weak monoprotic (monobasic) acids.
Scope: weak-acid and weak-base work will not call for quadratics - Track how pH moves through an acid-base titration and tie those movements to how strong the acid and the base are.
- With suitable data to hand, justify which indicator suits a particular acid-base titration.
- Explain the mechanism by which a buffer keeps pH steady.
- Describe where buffers are put to use, including how the CO32-/HCO3- pair steadies pH in oceans and how surging atmospheric carbon dioxide has driven ocean acidification.
- Given the necessary data, calculate a buffer's pH.
How it's examined
Questions on this topic most often ask you to sketch, describe, determine, explain. About 5% of the past-paper style questions in Rae's bank for this subject sit in this topic.
More worked questions on this topic
- Isopropylamine, (CH3)2CHNH2, is a weak base. Describe two laboratory tests, together with the r (2 marks)
- Draw a sketch of how pH changes with volume as 40.00 cm3 of 0.25 mol dm⁻³ HCl(aq) is added slow (2 marks)
- A pyridine buffer was proposed to keep the pH of an organic reaction mixture steady (pKb of pyr (2 marks)
- (b) Assume for this part that the acid is monobasic and can be written HA. A student wants to f (2 marks)
- A 250 cm³ buffer was made by mixing 120 cm³ of 0.100 mol dm⁻³ aqueous sodium hydroxide into 130 (2 marks)
Continue revising
- H2 Chemistry mole calculations: limiting reagents and gas quantities
- H2 reaction kinetics: initial rates, rate constants and half-life
- H2 equilibrium: Kc expressions, reaction tables and changing conditions
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry