A student carries out a titration using phenolphthalein indicator to determine the volume of sulfuric acid needed to neutralise a solution of potassium hydroxide. The student obtains a rough titre and then repeats the titration to obtain concordant results. Explain why the student carries out multiple titrations and state what is done with the concordant results to obtain a reliable final 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).

Model answer (4 marks)

The student performs several titrations to check reproducibility and to detect any anomalous runs. The first, rough titre gives an approximate end‑point, allowing the subsequent titrations to be carried out more accurately and the indicator added dropwise near the end‑point. The results that agree closely (within about 0.10 cm³) are called concordant. These concordant titres are then averaged – the mean is calculated – and this mean value is taken as the reliable final titre. The rough, preliminary titre is not included in the average.

Examiner tips

  • Show the purpose of the rough titre and the need for accuracy near the end‑point.
  • Explain that concordant results are those within a small range and that the mean of these is used.
  • Mention that the rough titre is excluded from the calculation.

Common mistakes

  • Confusing the rough titre with the final titre; not excluding it from the average.
  • Failing to state that concordant results must be within a small range (e.g. 0.10 cm³).
  • Not explaining why multiple titrations are needed for reproducibility.

Mark scheme (4 marks)

  1. Multiple titrations are carried out to check reproducibility / identify anomalous results
  2. The rough titre is used to identify the approximate end-point so that the final titrations can be performed more accurately / dropwise near the end-point
  3. Concordant results are titres that agree closely with each other (within 0.10 cm³ / accept within a small range)
  4. The concordant results are averaged (mean calculated) to obtain the final titre; the rough titre is excluded from this calculation

Key terms in this question

titration · concordant results · rough titre

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