# A student carries out thin-layer chromatography (TLC) to analyse a mixture of organic compounds. After running the chromatogram, the student measures the distances travelled by the solvent front and by each spot. Explain how the student uses these measurements to identify whether one of the compounds in the mixture is the same as a known reference compound, and describe one limitation of using TLC alone to confirm the identity of an unknown organic compound.

> WJEC A-Level Chemistry (Wales) — 4.8 Organic synthesis and analysis · Explain · 5 marks

## Mark scheme (5 marks)

1. The student divides the distance travelled by the compound (spot) by the distance travelled by the solvent front to calculate the Rf value
2. If the Rf value of the compound matches the Rf value of the reference compound (in the same solvent), the two compounds are likely to be the same
3. Different compounds can have the same Rf value in a given solvent, so matching Rf values do not guarantee the compounds are identical
4. A different solvent (or solvents) should be used and if the Rf values still match, this provides stronger evidence of identity
5. An additional analytical technique (such as mass spectrometry or infrared spectroscopy) should be used alongside TLC to confirm identity

## Key terms

- [thin-layer chromatography](https://www.gradenine.co.uk/glossary/thin-layer-chromatography)
- [solvent front](https://www.gradenine.co.uk/glossary/solvent-front)
- [reference compound](https://www.gradenine.co.uk/glossary/reference-compound)

## 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-student-carries-out-thin-layer-chromatography-c54a39d2) · Published by Druglandscape Ltd.