A weak-acid/strong-base curve has an equivalence volume of 25.00 cm3. At 12.50 cm3, the pH is 4.76. Determine pKa and Ka.
[3 marks]
Total for this question: 3
A-level Chemistry required practicals
pH curves for weak/strong acid–base reactions
AQA 3.1.12.5 · RP9
A-level Chemistry (7405) · Required practical 9 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
Measure and interpret pH curves for a weak acid with a strong base and a strong acid with a weak base.
Apparatus
- Calibrated pH meter and probe
- Burette, stand and funnel
- Pipette or measuring cylinder
- Beaker, magnetic stirrer and stir bar
- Weak acid, strong acid, strong base and weak base solutions
- Buffer solutions, distilled water and eye protection
Apparatus & techniques (AT)
AT a · measurement
Measures starting portions and titrant volumes at suitable precision.
AT c · pH measurement
Calibrates and uses a pH probe to follow both acid–base reactions.
AT d · titration/distillation/reflux/qualitative tests/filtration
Adds titrant from a burette and mixes each acid–base system before recording pH.
AT k · safe handling
Handles acidic and alkaline solutions safely and rinses spills promptly.
Safety
Hazard
Acid and alkali solutions can irritate or burn skin and eyes.
Control
Wear eye protection, fill the burette below eye level and rinse splashes immediately with plenty of water.
Hazard
Aqueous ammonia used in the weak-base pairing gives off an irritant vapour.
Control
Work in a fume cupboard or well-ventilated area and keep the ammonia bottle stoppered when not dispensing.
Method
- 1Calibrate the pH meter with suitable buffer solutions, rinse the probe with distilled water and blot it without rubbing.
- 2Weak-acid/strong-base branch: measure a fixed volume of weak acid into the beaker and add strong base from the burette while stirring.
- 3Strong-acid/weak-base branch: use a fresh measured strong-acid portion and repeat with weak base as the titrant.
- 4After each addition, allow the reading to stabilise and record total titrant volume and pH; use smaller volume increments through the region where pH changes rapidly.
- 5Continue beyond equivalence, rinse the apparatus between pairings and keep concentration, starting volume and temperature controlled for comparisons.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 2: Editorial focus: choose volume increments that resolve each curve's rapid-change region.
- CPAC 4: Editorial focus: record stable pH and cumulative volume for both specified pairings.
- CPAC 5: Editorial focus: extract equivalence, pKa/Ka and indicator suitability from the curves.
Variables
Independent
Total volume of titrant added
Dependent
Equilibrated pH of the reaction mixture
Control
- Acid and base concentrations
- Starting acid volume
- Temperature, stirring and probe-calibration procedure
Results & processing
- Plot pH against total titrant volume for each pairing and locate the equivalence region from the steepest change.
- For the weak-acid/strong-base curve, use pH = pKa at half-equivalence and calculate Ka = 10−pKa.
- Choose an indicator only if its transition range lies within the curve's rapid-change region.
Analysis skills
- Plot and compare pH–volume curves for both specified pairings.
- Use the half-equivalence point to determine pKa and Ka for a weak acid.
- Match an indicator transition range to the steep region of a measured curve.
Uncertainty
Sources
- Burette reading resolution
- pH-probe calibration, resolution and drift
- Finite titrant increments and incomplete equilibration
Calculations
- A delivered volume is the difference of two burette readings, so include both reading uncertainties.
- Report equivalence volume as an interval bounded by the fine additions around the steepest change.
Interpretation
- Smaller increments improve equivalence-volume resolution but increase the number of readings and time for drift.
- Calibration error shifts pH values systematically and is not removed by repeating the same uncalibrated run.
Exam angles
- Explain where to reduce titrant increments and link the choice to the changing pH.
- Determine pKa, Ka or equivalence volume from a graph.
- Choose and justify an indicator separately for each acid–base pairing.
Where students lose marks
Writing only ‘add the titrant dropwise’ without saying where or why smaller additions are needed.
Fix: Use coarse additions away from equivalence and fine increments where the recorded pH begins changing rapidly.
- Examiner report: C3-24 · PDF p. 5
Using pH = pKa at the equivalence point.
Fix: Apply that equality at half-equivalence for the weak-acid/strong-base pairing.
Selecting an indicator because its colour is familiar rather than using its transition range.
Fix: Superimpose the indicator range on the steep part of the relevant curve.
Improve the method
- Use a magnetic stirrer at constant speed and wait for a stable value after every addition.
- Take much smaller volume increments around the rapid-change region to narrow the equivalence-volume interval.
- Recalibrate the probe, rinse between readings and keep the bulb immersed without touching the beaker.
Source references
- Specification: CSpec 3.1.12.5 · PDF p. 37
Try it — exam-style
Medium
ORIGINAL
Easy
ORIGINAL
A strong-acid/weak-base curve changes rapidly from pH 4.1 to pH 6.2. Indicator X changes from pH 4.4–6.0 and indicator Y from pH 8.2–10.0. Which is suitable? Explain.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers — never copied from them.
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