# 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

> 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.

## 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

- [titre](https://www.gradenine.co.uk/glossary/titre)
- [mean titre](https://www.gradenine.co.uk/glossary/mean-titre)

## Related

- [Revision notes for Cambridge International IGCSE Chemistry (0620)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-carries-out-a-titration-705b2b45) · Published by Druglandscape Ltd.