Describe how a student would carry out an acid-base titration to find the volume of hydrochloric acid needed to neutralise 25.0 cm³ of sodium hydroxide solution, using an indicator. Include how the student would know when the end-point has been reached.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Use a pipette (and pipette filler) to measure 25.0 cm³ of sodium hydroxide solution into a conical flask.
Add a few drops of a suitable indicator (e.g. phenolphthalein) to the sodium hydroxide solution.
Fill a burette with hydrochloric acid and add it slowly, dropwise, to the conical flask while swirling.
The end‑point is reached when the indicator permanently changes colour (for phenolphthalein the pink colour disappears to colourless).
Add a few drops of a suitable indicator (e.g. phenolphthalein) to the sodium hydroxide solution.
Fill a burette with hydrochloric acid and add it slowly, dropwise, to the conical flask while swirling.
The end‑point is reached when the indicator permanently changes colour (for phenolphthalein the pink colour disappears to colourless).
Examiner tips
- Use a pipette for accurate 25.0 cm³ measurement.
- Add the indicator before titration starts.
- Add acid slowly and swirl to mix.
- Identify the permanent colour change of the chosen indicator.
Common mistakes
- Using a graduated cylinder instead of a pipette for 25.0 cm³.
- Adding acid too quickly, causing overshoot.
- Not recognising the permanent colour change as the end‑point.
Mark scheme (4 marks)
- Use a pipette (and pipette filler) to measure 25.0 cm³ of sodium hydroxide solution into a conical flask
- Add a few drops of a named indicator to the sodium hydroxide solution in the conical flask
- Fill a burette with hydrochloric acid and add it slowly / dropwise to the conical flask, swirling throughout
- End-point is when the indicator permanently changes colour (and the change is consistent with a named indicator, e.g. phenolphthalein changes from pink to colourless)
Key terms in this question
titration · end-point · indicator
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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