A Level · Cambridge (CIE) · 9701

Calculate and interpret an Rf value

Calculate Rf as the spot distance divided by the solvent-front distance, both measured from the baseline.

Analytical techniques · Cambridge International AS & A Level Chemistry 9701 · 37.1

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?

Put the idea to work

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.

Try a topic test

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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.

1 · Guided

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

2 · Independent

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

3 · Transfer

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

Recall without your notes

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.

Keep learning Chemistry

Check the official syllabus 2028–2030

Course scope and assessment

Four hundred and forty-two original lessons map the published learning outcomes across AS Chemistry Topics 1–22 and A Level Topics 23–37, with guided, independent and transfer practice and explained answers. Every numbered subject-content topic and listed outcome tag now has a mapped lesson sequence. Topic 32 maps all 12 hydroxy-compound outcome tags in 11 lessons; Topic 33 maps all 15 carboxylic-acid and derivative outcome tags in 16 lessons; Topic 34 maps all 22 nitrogen-compound outcome tags in 20 lessons; Topic 35 maps all 12 polymerisation outcome tags in 16 lessons; Topic 36 maps all four organic-synthesis outcome tags in 14 lessons; Topic 37 maps all 22 chromatography and NMR outcome tags in 18 lessons. Cambridge's 2028–2030 Version 1 syllabus states that there are no significant changes which affect teaching from the preceding edition; the routes are aligned to that edition. This is 37 of the 37 numbered subject-content topics. Full cumulative exam assessment, Paper 3 laboratory competence, Paper 5 investigation practice and human subject-teacher review remain outstanding. Written practice is self-checked and does not certify exam readiness.