A student carries out a titration to find the concentration of a solution of sodium hydroxide. The student uses sulfuric acid of known concentration and methyl orange indicator. Explain why the results from the first (rough) titration are not included when calculating the mean titre.

Cambridge International IGCSE Chemistry (0620) — 12.2 Acid-base titrations · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In a titration, a rough titration is always performed before the accurate runs. The rough titre is used only as a guide for the accurate titrations that follow.

Model answer (4 marks)

The rough titration is only a guide. It is performed quickly and the acid is added in larger drops, so the student cannot stop at the exact colour change. Consequently the volume recorded is usually larger and less precise than the accurate titres. If this value were included in the mean, it would skew the result and give a less accurate concentration.

Examiner tips

  • Mention that the rough titre is not accurate because of rapid addition and lack of dropwise technique.
  • Explain that including it would bias the mean towards a higher volume, reducing accuracy.
  • Show the logical link between inaccuracy and exclusion from the mean.
  • Use the exact terms: rough titre, accurate titres, mean titre, concentration.

Common mistakes

  • Saying the rough titre is correct and should be used.
  • Failing to explain why the rough titre is less precise (e.g., not mentioning rapid addition).
  • Using vague language like "not accurate" without linking to the mean calculation.

Mark scheme (4 marks)

  1. The rough titration gives an approximate endpoint / rough titre value
  2. The student adds the acid / alkali too quickly (does not add dropwise near the endpoint) in the rough titration
  3. This means the rough titre is likely to be larger / less precise than the accurate titres
  4. Including the rough titre would increase / skew the mean titre, making the calculated concentration less accurate

Key terms in this question

titre · mean titre

Related

More Acid-base titrations questions

▶ Try answering this question with AI marking (free) →