Calculate and interpret an Rf value
Calculate Rf as the spot distance divided by the solvent-front distance, both measured from the baseline.
Section 37 · Analytical techniques
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 37. Use the supplied spectra, shift table and chromatography conditions. Interpret peaks as evidence and state when a short trace cannot uniquely identify a substance. Written practice is self-checked
Before you begin
- Recognise functional groups, molecular structures, polarity, intermolecular interactions and the distinction between equivalent and different chemical environments.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which formula defines Rf?
What you will learn
- Use the Rf definition
- Calculate dimensionless Rf values
- Check the result against the solvent-front distance
Use one common origin
Rf = distance travelled by the centre of a substance spot from the baseline ÷ distance travelled by the solvent front from the baseline. Use the same units for both distances so they cancel.
Rf has no unit. If the spot travels 2.4 cm and the front 6.0 cm, Rf = 2.4/6.0 = 0.40.
Check what the number means
For a spot that remains behind the solvent front, Rf lies between 0 and 1. A larger Rf means the spot travelled farther relative to the front under those conditions; it does not directly state the molecule's size or concentration.
Rf comparisons are meaningful when the stationary phase, solvent and other conditions are the same. Different solvents can change the value for the same substance.
Pause and explain
A spot travels 4.2 cm while the front travels 6.0 cm. What is Rf?
Worked example
A spot travels 3.5 cm and the solvent front travels 7.0 cm from the baseline. Calculate Rf.
Show the worked solution
- Write the spot distance in the numerator.
- Divide by the front distance.
- Cancel centimetres and omit units.
Answer Rf = 3.5/7.0 = 0.50.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A component moves 1.8 cm and the solvent front moves 6.0 cm. Find Rf.
Give me a hint
Divide the spot distance by the front distance.
Compare my reasoning
- Use 1.8 in the numerator.
- Use 6.0 in the denominator.
- The centimetres cancel.
Rf = 1.8/6.0 = 0.30.
Look for these in your work
- Correct order
- Correct calculation
- No unit
A spot has Rf = 0.65 and the solvent front travelled 8.0 cm. How far did the spot travel?
Give me a hint
Rearrange Rf = spot distance/front distance.
Compare my reasoning
- Spot distance = Rf × front distance.
- Multiply 0.65 by 8.0 cm.
- Retain a distance unit.
0.65 × 8.0 cm = 5.2 cm.
Look for these in your work
- Equation rearranged
- Product correct
- Unit retained
A calculated TLC value is Rf = 1.18 even though the spot appears below the solvent front. What should you check?
Give me a hint
Compare the numerator and denominator measurements.
Compare my reasoning
- The spot distance should not exceed the front distance.
- Check whether both were measured from the baseline.
- Check for a reversed ratio or measurement error.
Recheck the distances and ratio: for a spot below the front, spot distance should be less than the front distance, so Rf should be below 1.
Look for these in your work
- Range checked
- Common origin checked
- Likely error identified
Common mistakes
- Inverting the ratio
- Adding the two distances
- Reporting centimetres as the unit of Rf
- Comparing values obtained with different solvents as if conditions were identical
What can you explain now?
Which formula defines Rf?
Compare with the explanation
Spot distance ÷ solvent-front distance
Both distances are measured from the baseline.
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Make it stick
How do you calculate Rf, and what does its relative size tell you?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.