# A scientist is testing two painkiller tablets from different manufacturers. Both tablets claim to contain the same active ingredient. The scientist uses paper chromatography with the same solvent to test samples from each tablet alongside a reference sample of the pure active ingredient. The pure active ingredient produces a single spot. Tablet A produces a single spot at the same position as the reference. Tablet B produces two spots, one of which is at the same position as the reference. Explain what the chromatography results tell the scientist about each tablet, and describe how the Rf value could be used to help confirm the identity of the active ingredient.

> WJEC A-Level Chemistry (Wales) — 1.2 Electronic structure · Explain · 5 marks

## Mark scheme (5 marks)

1. Tablet A contains only one substance / is a pure substance
2. Tablet A's spot is at the same position as the reference, so it contains the active ingredient
3. Tablet B is a mixture / contains more than one substance
4. Tablet B contains the active ingredient AND another substance / an additional compound
5. The Rf value is calculated as distance travelled by the substance divided by the distance travelled by the solvent, and each compound has a characteristic Rf value in a given solvent which can be compared to known reference values to confirm identity

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-is-testing-two-painkiller-387cabde) · Published by Druglandscape Ltd.