Start a short session
Check your answer after each question. Use a calculator where needed.
Pigment separation · distances · ratios
Chromatography and Rf calculations
Calculate Rf, pigment spot distance and solvent-front distance. Use matching units and distances measured from the same origin.
Method and worked examples ↓Learn the method
How to approach these questions
- Measure the spot and solvent-front distances from the same origin line.
- Convert distances to matching units before dividing.
- To find solute distance, multiply Rf by solvent-front distance. To find solvent-front distance, divide solute distance by Rf.
- A spot between origin and solvent front gives a ratio between 0 and 1.
- Compare Rf values only under comparable conditions. Matching values can support an identification, but do not prove that two substances are identical.
Worked examples
A spot centre travels 36 mm and the solvent front 60 mm from the origin. Find Rf.
Rf = 36/60 = 0.60. Millimetres cancel, leaving a dimensionless ratio.
A pigment has Rf 0.40 in a chromatogram whose solvent front travels 75 mm. Find the expected spot distance.
Spot distance = 0.40 × 75 = 30 mm from the same origin.
Keep practising
Topic map and study list · Practical data guides · Active recall decks
Course resources
AQA A-level Biology · OCR A A-level Biology · Textbook answers
Skill coverage follows the common A-level science subject content. Check your board’s current requirements: AQA 7402 and OCR A H420.