This worked example covers Rf Value Calculations within Chromatography for GCSE Chemistry. Revise Chromatography in Chemical Analysis for GCSE Chemistry with 23 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 6 of 13 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.

🧮 Rf Value Calculations

Formula:

Rf = distance moved by substance ÷ distance moved by solvent front

Key Properties

Always between 0 and 1 (no units — it is a ratio)
Unique to each substance under specific solvent conditions
Only valid for comparison if the same solvent and paper are used

Worked Example

Given: Solvent front moved 8.0 cm. Substance A moved 6.4 cm. Substance B moved 2.4 cm.

Rf (A) = 6.4 ÷ 8.0 = 0.80

Rf (B) = 2.4 ÷ 8.0 = 0.30

Measuring Distances

Always measure from the start line
For substances: measure to the centre of each spot
For solvent: measure to the solvent front

Quick Check: The solvent front travels 10.0 cm. A substance spot travels 7.5 cm. Calculate the Rf value.

Practice questions for Chromatography

What is the purpose of chromatography?

A. To separate the components of a mixture
B. To measure the mass of a substance
C. To change a substance from a solid to a liquid
D. To make a substance more concentrated

1 markfoundation

Explain how a chromatogram can be used to determine whether a substance is pure or a mixture.

3 marksstandard

Quick recall flashcards

What does Rf stand for and what does it measure?

Rf = Retention factor. It measures how far a substance travels compared to the solvent (distance moved by substance ÷ distance moved by solvent)

What is chromatography?

A separation technique used to separate mixtures into their individual components based on different affinities for mobile and stationary phases

Rf Value Calculations