Calculate the pH of 0.0500 mol dm−3 hydrochloric acid.
[1 mark]
Total for this question: 1
Chemistry
Acids and bases
AQA 3.1.12
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
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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. Strong-acid/strong-base curves usually have the largest steep pH change, but strong/weak combinations also have steep regions shifted away from pH 7; weak-acid/weak-base curves have no sharp vertical region.
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
Easy
ORIGINAL
Hard
ORIGINAL · after June 2023 Paper 1 Q10.3
Propanoic acid has Ka = 1.35 x 10−5 mol dm−3. Calculate the pH of a 0.100 mol dm−3 solution.
[3 marks]
Total for this question: 3
Medium
ORIGINAL · after June 2024 Paper 1 Q07.1
Calculate the pH of 0.100 mol dm−3 sodium hydroxide at 298 K. (Kw = 1.0 x 10−14 mol2 dm−6)
[2 marks]
Total for this question: 2
Hard
ORIGINAL · after June 2023 Paper 1 Q10.4
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.
[3 marks]
Total for this question: 3
Medium
ORIGINAL · after June 2023 Paper 1 Q10.5
Explain why no indicator is suitable for a titration between a weak acid and a weak base.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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