# A chemist is analysing an unknown organic compound using thin-layer chromatography (TLC). The chemist runs two samples side by side on the TLC plate: the unknown compound and a reference standard. After development, the chemist calculates the Rf values of the spots and compares them. Explain how the chemist can use the results of this TLC experiment to determine whether the unknown compound is the same as the reference standard, and describe ONE limitation of using Rf values alone to confirm the identity of a compound.

> OCR A-Level Chemistry B: Salters (H433) — 6.3 Analysis (chromatography, NMR) · Explain · 5 marks

> Thin-layer chromatography (TLC) is an analytical technique used to separate and identify the components of a mixture. Each component travels a characteristic distance relative to the solvent front, giving a value called the Rf value.

## Mark scheme (5 marks)

1. The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
2. If the unknown compound has the same Rf value as the reference standard, this suggests they may be the same compound
3. If the Rf values are different, the unknown compound is not the same as the reference standard
4. A limitation is that two different compounds can have the same Rf value in a given solvent, so a match does not confirm identity with certainty
5. A further technique (e.g. running the chromatogram in a different solvent, or using an additional analytical technique) would be needed to confirm identity

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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-chemist-is-analysing-an-unknown-2cf6698f) · Published by Druglandscape Ltd.