AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Chromatography: R_f = distance moved by substance / distance moved by solvent
This equation turns a chromatogram into a number.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation turns a chromatogram into a number. Distance moved by substance means from the pencil origin line to the centre of the spot; distance moved by solvent means from the same origin line to the solvent front. Suppose the solvent front is 10.0 cm from the origin and a spot centre is 4.5 cm from the origin: R_f = 4.5 ÷ 10.0 = 0.45. The value has no units because it is a length divided by a length, and it is always 1 or less. Rearranging helps too: distance moved by substance = R_f × distance moved by solvent. Use the equation with a reference substance on the same chromatogram to identify an unknown, since R_f changes with solvent and conditions.
What earns the marks
- Substitute the distance moved by the substance and the distance moved by the solvent into the equation in the correct positions.
- Keep the substance distance as the numerator and the solvent distance as the denominator.
- Measure both distances from the origin line to the centre of the spot and to the solvent front respectively.
- Report R_f as a unitless value, usually between 0 and 1, to an appropriate number of significant figures.
- Rearrange the equation to find an unknown distance when the R_f value and the other distance are given.
- Use R_f values only for comparison when the reference and unknown were run on the same chromatogram with the same solvent.
Common mistakes and how to put them right
Inverting the fraction and dividing the solvent distance by the substance distance.
Put it right by writing the equation in words before substituting numbers.
Adding units such as cm to the R_f answer.
Put it right by cancelling the length units, since a length is divided by a length.
Using a spot distance greater than the solvent distance and reporting R_f above 1.
Put it right: Re-measuring, as the spot cannot move further than the solvent front.
Exam tips
- Show the substitution line, for example R_f = 4.5 ÷ 10.0, before giving the final value so method marks can be awarded.
- Round the final R_f sensibly, such as two decimal places, and avoid over-long calculator displays.
- If asked to identify a substance, quote both the calculated R_f and the matching reference R_f from the same chromatogram.
Try the lesson
The start of the Chromatography: R_f = distance moved by substance / distance moved by solvent lesson, made from checked teaching notes for this specification point.
Have a go first
In a paper chromatography experiment, a spot of food colouring moves 2.5 cm from the start line. The solvent front moves 10.0 cm from the start line. What is the R_f value of the food colouring?
Listen
This equation turns a chromatogram into a number. Distance moved by substance means from the pencil origin line to the centre of the spot; distance moved by solvent means from the same origin line to the solvent front. Suppose the solvent front is 10.0 cm from the origin and a spot centre is 4.5 cm from the origin: R_f = 4.5 ÷ 10.0 = 0.45. The value has no units because it is a length divided by a length, and it is always 1 or less. Rearranging helps too: distance moved by substance = R_f × distance moved by solvent. Use the equation with a reference substance on the same chromatogram to identify an unknown, since R_f changes with solvent and conditions. A strong answer earns marks for these points. Substitute the distance moved by the substance and the distance moved by the solvent into the equation in the correct positions. Keep the substance distance as the numerator and the solvent distance as the denominator. Measure both distances from the origin line to the centre of the spot and to the solvent front respectively. Report R_f as a unitless value, usually between 0 and 1, to an appropriate number of significant figures. A common slip: Inverting the fraction and dividing the solvent distance by the substance distance. Instead, writing the equation in words before substituting numbers. Exam tip: Show the substitution line, for example R_f = 4.5 ÷ 10.0, before giving the final value so method marks can be awarded.
See it
Key words
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R_f
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Distance moved by substance
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Distance moved by solvent
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Rearranging
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See every definition
- R_f
- A value calculated as distance moved by substance divided by distance moved by solvent, which has no units and is always 1 or less.
- Distance moved by substance
- The length from the pencil origin line to the centre of the spot.
- Distance moved by solvent
- The length from the same origin line to the solvent front.
- Rearranging
- Changing the equation so that distance moved by substance equals R_f multiplied by distance moved by solvent.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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