OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 1

1 mark · Medium difficulty · Multiple Choice

Calculate the percentage uncertainty of a titre obtained from initial and final burette readings, given the uncertainty of each reading.

Practise this question

Question

Multiple-choice question showing a table with titration readings: Initial reading equals 4.95 cm³ and Final reading equals 23.10 cm³. The text states that the burette has an uncertainty of 0.05 cm³ for each reading and asks for the percentage uncertainty of the resulting titre. Four options are given: A 0.22%, B 0.28%, C 0.44%, and D 0.55%.
Question text

1 The burette readings from a titration are shown below.

Initial reading / cm3 4.95

Final reading / cm3 23.10

The burette used has an uncertainty of 0.05 cm3 for each reading.

What is the percentage uncertainty of the resulting titre?

A 0.22%

B 0.28%

C 0.44%

D 0.55%

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme snippet showing Question 1 with correct answer D and a mark value of 1.

1 D 1

How to answer it

Burette Titre & Percentage Uncertainty

📋 What this question tests

This question assesses practical experimental skills from Module 1: Development of practical skills in chemistry:

  • Calculating a titre from initial and final burette readings.
  • Understanding that determining a titre requires two separate readings, doubling the absolute reading uncertainty.
  • Calculating percentage uncertainty using: percentage uncertainty = (total uncertainty ÷ measured value) × 100% .

Question 1

Multiple Choice (1 Mark)

✅ Correct Answer

D  (0.55%)

📐 Step-by-Step Calculation

  1. Calculate the titre volume:
    Titre = Final reading − Initial reading
    Titre = 23.10 cm³ − 4.95 cm³ = 18.15 cm³
  2. Determine the total absolute uncertainty:
    A titre requires two independent readings (initial and final), each with an uncertainty of ±0.05 cm³.
    Total uncertainty = 2 × 0.05 cm³ = 0.10 cm³
  3. Calculate percentage uncertainty:
    Percentage uncertainty = (Total uncertainty ÷ Titre volume) × 100%
    Percentage uncertainty = (0.10 cm³ ÷ 18.15 cm³) × 100% = 0.55096...%
    To 2 significant figures = 0.55%

💡 Key Knowledge

  • Measurement vs Reading: A single reading (e.g. reading a volumetric flask) has 1 uncertainty. A difference between two readings (burette titre, mass by difference, temperature change) has 2 uncertainties.
  • Rule for adding errors: Absolute uncertainties always add together when values are subtracted: Δ(V₂ − V₁) = ΔV₁ + ΔV₂ .
  • Standard burette reading uncertainty is ±0.05 cm³, giving an overall titre uncertainty of ±0.10 cm³.

🧠 Exam Technique & Options Breakdown

  • Option A (0.22%): Trap where the candidate used only one uncertainty divided by final volume: (0.05 ÷ 23.10) × 100 .
  • Option B (0.28%): The most frequent trap! The candidate calculated the titre correctly but forgot that there are two readings: (0.05 ÷ 18.15) × 100 .
  • Option C (0.44%): Candidate remembered to multiply by 2, but divided by the final reading rather than the titre: (0.10 ÷ 23.10) × 100 .
  • Option D (0.55%): Correct application of both readings and the titre volume.

❌ Common Student Errors

  • Forgetting to multiply by 2: Examiners report this as the single most common mistake in titration uncertainty calculations. Remember: taking a titre is always a subtraction of two readings.
  • Dividing by the final reading: Always divide uncertainty by the quantity being calculated (the titre = 18.15 cm³), never by the single meniscus position on the burette.
Mark Scheme Note: Award [1] mark for selecting option D. No partial marks are available for multiple-choice questions.

Topics

Module 1: Development of practical skills in chemistry · Practical Activity Groups · 1.1 Practical skills assessed in a written examination · PAG 2: Acid-base titration

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.