AQA A-Level Chemistry AS Paper 1, June 2023: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify which suggestion decreases the percentage uncertainty in the mean titre during an acid-base titration.

Practise this question

Question

Multiple choice question labelled 11, asking: 'Which suggestion decreases the percentage uncertainty in the mean titre?' It offers four options: A, 'Use a more dilute solution of sodium hydroxide in the burette'; B, 'Use a more dilute solution of vinegar'; C, 'Rinse the inside of the conical flask with distilled water during each titration'; D, 'Rinse the tip of the burette with distilled water near the end point in each titration'. It is worth 1 mark.
Question text

11 Which suggestion decreases the percentage uncertainty in the mean titre?

[1 mark]

A Use a more dilute solution of sodium hydroxide in the burette.

B Use a more dilute solution of vinegar.

Rinse the inside of the conical flask with distilled water during each

C

titration.

Rinse the tip of the burette with distilled water near the end point in

D

each titration.

Mark scheme

Show the mark scheme Mark scheme excerpt for question 11 displaying answer 'A', corresponding to 'Use a more dilute solution of sodium hydroxide in the burette', worth 1 mark under AO3.

11 A 1 (AO3) Use a more dilute solution of sodium hydroxide in the burette.

How to answer it

AQA A-Level Chemistry • Practical Skills & Apparatus

Reducing Percentage Uncertainty in Titration

What this question tests

  • Understanding the formula for percentage uncertainty: percentage uncertainty = (uncertainty / reading) × 100 .
  • Evaluating modifications to experimental design to decrease percentage apparatus uncertainty.
  • Distinguishing between practical techniques that improve accuracy (eliminating systematic error) versus those that reduce percentage uncertainty.

Question 11 (Multiple Choice)

Decreasing Percentage Uncertainty in Titre Volume

✅ Correct Answer: A

Use a more dilute solution of sodium hydroxide in the burette.

Mark allocation: 1 mark (AO3) for identifying option A.

💡 Key Knowledge

  • Percentage Uncertainty Formula:
    % uncertainty = (2 × apparatus uncertainty / titre volume) × 100
  • Because the apparatus uncertainty (e.g. ±0.05 cm³ per burette reading, total ±0.10 cm³) is fixed, the only way to decrease percentage uncertainty is to increase the volume of the titre measured.
  • If the concentration of the standard solution in the burette (NaOH) is decreased, a larger volume of NaOH is required to neutralise the same fixed amount of acid (vinegar) in the flask:
    volume = moles / concentration

📐 Step-by-Step Reason Why A Works

  1. Moles of ethanoic acid in the flask remain constant: n = c × V .
  2. Reaction stoichiometry is 1:1:
    CH₃COOH + NaOH → CH₃COONa + H₂O
  3. Therefore, moles of NaOH needed remain constant.
  4. Lowering the concentration of NaOH ( c ) causes the volume required ( V = n / c ) to increase.
  5. With a larger titre volume in the denominator, the percentage uncertainty (ΔV / V) × 100 decreases.

❌ Common Errors & Distractor Breakdown

  • Why B is incorrect: Using a more dilute solution of vinegar means fewer moles of acid in the aliquot. This would require a smaller volume of NaOH, which would increase the percentage uncertainty.
  • Why C and D are incorrect: Rinsing the conical flask walls or burette tip with distilled water ensures all reactants enter the mixture and react. This improves accuracy (reduces procedural errors), but it has no impact on the apparatus percentage uncertainty of the titre.

🧠 Exam Technique & Examiner Insight

  • Watch the wording: Questions asking to decrease uncertainty are almost always testing your ability to make the measured quantity larger.
  • Distinguish accuracy vs. uncertainty: Many students fall into the trap of picking C or D because they remember them as good laboratory titration habits. Always ask yourself: "Does this change the numeric value on the burette scale?"
  • If asked how to modify the flask contents to decrease burette uncertainty, the answer would be to use a larger pipette volume (e.g. 50 cm³ instead of 25 cm³) or more concentrated vinegar.

Topics

Physical Chemistry · Required Practicals · 3.1.2 Amount of Substance · Required Practical 1: Making up a volumetric solution

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.