OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 1

1 mark · Medium difficulty · Multiple Choice

Identify which experimental procedure explains why a student obtains a smaller titre during an acid-base titration of sodium hydroxide with hydrochloric acid.

Practise this question

Question

Multiple choice question 1 asking which procedure explains a smaller titre when titrating sodium hydroxide with hydrochloric acid. Four options are given: A, burette readings from the top of the meniscus; B, conical flask rinsed with water; C, air bubble released from the burette jet; D, pipette rinsed with water before filling with sodium hydroxide. An answer box and mark allocation [1] are shown at the bottom.
Question text

1 Several students titrate 25.00 cm3 of the same solution of sodium hydroxide, NaOH(aq) with

hydrochloric acid, HCl(aq).

One student obtains a smaller titre than the other students.

Which procedure explains the smaller titre?

A The burette readings are taken from the top of the meniscus instead of the bottom of the

meniscus.

B The conical flask is rinsed with water before carrying out the titration.

C An air bubble is released from the jet of the burette during the titration.

D The pipette is rinsed with water before filling with NaOH(aq).

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table showing question number 1 with the correct answer D, worth 1 mark and mapped to AO element 2.7.

AO

Question Answer Marks Guidance

element

1 D 1 2.7

How to answer it

Evaluating Titration Errors and Practical Techniques

What this question tests

This question assesses your understanding of practical chemistry techniques, specifically titration procedures using volumetric glassware (pipettes and burettes). It tests your ability to predict how experimental errors and incorrect rinsing procedures affect the final titre volume of an acid-base neutralization reaction between sodium hydroxide ( NaOH ) and hydrochloric acid ( HCl ).

Multiple Choice Question 1

Analysis of Titration Procedures

✅ Correct Answer: D

Rinsing the pipette with water before filling it with NaOH(aq) introduces residual water, which dilutes the sodium hydroxide solution. This means fewer moles of NaOH are actually delivered into the conical flask, requiring a smaller volume of HCl to neutralize it, hence resulting in a smaller titre.

💡 Key Knowledge: Glassware Rinsing Rules

  • Pipette & Burette: Rinse with the solution they are going to contain (to avoid dilution).
  • Conical Flask: Rinse only with distilled/deionised water (does not change the number of moles inside, only the concentration, which doesn't affect stoichiometry).

🧠 Exam Technique

When tackling "smaller/larger titre" questions, always ask yourself: "Does this error change the number of moles of the reactant in the flask?" Trace how a change in moles directly forces a change in the volume of acid added from the burette.

❌ Common Errors & Distractor Analysis

  • A (Top of meniscus): Taking readings from the top instead of the bottom consistently shifts both initial and final readings by the same offset, so the difference (titre) remains largely unaffected.
  • B (Rinsing conical flask): Adding water to the flask changes the total volume but not the moles of NaOH inside. Titre remains unchanged!
  • C (Air bubble in jet): If an air bubble fills and then releases during titration, the recorded titre will be larger because some of the measured volume went into filling the jet rather than reacting.
Mark Scheme Allocation: 1 mark awarded for selecting option D (Assessment Objective 2.7: Applying investigative skills and practical procedures).

Topics

Module 2: Foundations in chemistry · Practical Activity Groups · PAG 2: Acid-base titration · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.