# A chemist uses thin-layer chromatography (TLC) to investigate a sample of an unknown organic compound. After running the chromatogram, only one spot is visible. The chemist then calculates the Rf value of this spot and compares it to values in a reference table of known compounds. Explain how thin-layer chromatography separates the components of a mixture and describe how the Rf value of a spot is determined and used to identify an unknown compound.

> OCR A-Level Chemistry A (H432) — 6.3 Analysis (chromatography, NMR) · Explain · 5 marks

> Thin-layer chromatography is widely used in organic chemistry laboratories to check the purity of compounds and to identify unknown substances by comparing their Rf values to those of known reference compounds.

## Mark scheme (5 marks)

1. The mobile phase (solvent) carries components of the mixture up the TLC plate
2. Different components travel different distances because they have different affinities/attractions for the stationary phase and mobile phase
3. Rf value = distance travelled by spot ÷ distance travelled by solvent front
4. The Rf value of the unknown spot is compared to Rf values of known/reference compounds (under the same conditions)
5. A single spot on the chromatogram indicates the sample is pure (contains only one component)

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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-chemist-uses-thin-layer-chromatography-tlc-d24e5465) · Published by Druglandscape Ltd.