This worked example covers Rf Value Calculations within Chromatography for GCSE Chemistry. Revise Chromatography in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. 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.
Topic position
Section 6 of 13
Practice
20 questions
Recall
15 flashcards
🧮 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.
Rf = 7.5 ÷ 10.0 = 0.75