Acids and bases
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
The topic on one screen
- pH = -log[H+], and [H+] = 10−pH. A strong acid fully dissociates, so [H+] = the acid concentration.
- Weak acids only partially dissociate: Ka = [H+][A-] / [HA]; for a weak acid alone, [H+] = sqrt(Ka x [HA]).
- Kw = [H+][OH-] = 1.0 x 10−14 at 298 K. For a strong base, find [OH-] first, then [H+] = Kw / [OH-].
- A buffer is a weak acid plus its conjugate base; it resists pH change by removing added H+ or OH-.
- Buffer pH: react the base with the acid FIRST, then [H+] = Ka x [weak acid left] / [salt formed].
- Give pH answers to 2 decimal places, and remember the strong-acid/strong-base titration is the only one with a long vertical section.
Where students actually lose marks
In the buffer calculation many did not subtract the moles of NaOH from the moles of weak acid to find the excess acid. React them first, then use the leftover acid and the salt formed.
June 2023 Paper 1 examiner report (Q10.4)
Explanations of why no indicator suits a weak-acid/weak-base titration were poor: the key point is the pH changes gradually, so there is no steep (vertical) section for an indicator to change over.
June 2023 Paper 1 examiner report (Q10.5)
In the Kw/pH calculation, common errors were forgetting to take the square root and not giving the answer to 2 decimal places. Watch both.
June 2024 Paper 1 examiner report (Q07.1)
Try it — exam-style
Calculate the pH of 0.0500 mol dm−3 hydrochloric acid.
Propanoic acid has Ka = 1.35 x 10−5 mol dm−3. Calculate the pH of a 0.100 mol dm−3 solution.
Calculate the pH of 0.100 mol dm−3 sodium hydroxide at 298 K. (Kw = 1.0 x 10−14 mol2 dm−6)
A buffer is made by adding 0.0200 mol NaOH to 0.0500 mol butanoic acid in 1.00 dm3 of solution. Ka(butanoic acid) = 1.51 x 10−5 mol dm−3. Calculate the pH.
Explain why no indicator is suitable for a titration between a weak acid and a weak base.
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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Buffer and weak-acid pH questions are pure method — get the order of steps right and they're free marks. I teach exactly that, and the first lesson is free.