Rf Value Calculations

Rf value calculations are used in paper chromatography to identify substances. The calculation is straightforward: it compares how far a substance has moved to how far the solvent front has moved. This is one of the simplest calculations in the IGCSE Chemistry syllabus, but accuracy in measurement is essential.

The method

1. Measure the distance from the baseline (pencil line) to the centre of the spot.
2. Measure the distance from the baseline to the solvent front.
3. Divide the spot distance by the solvent front distance.

The formula

Rf = distance moved by substance / distance moved by solvent front

Where:

• Both distances are measured from the same baseline (the pencil start line)
• The Rf value has no units and is always between 0 and 1

Worked examples

Example 1: Simple Rf calculation

Question: In a chromatography experiment, the solvent front moved 10 cm from the baseline. A spot of dye moved 7 cm. Calculate the Rf value.

Working:

• Rf = 7 / 10 = 0.70

Answer: Rf = 0.70

Examiner note: Give the Rf value to 2 decimal places unless told otherwise.

Example 2: Identifying an unknown

Question: A student ran chromatography on an unknown substance and three reference dyes. The solvent front moved 8.0 cm. The unknown spot moved 4.8 cm. Reference dye A moved 2.4 cm, B moved 4.8 cm, and C moved 6.0 cm. Which dye is the unknown?

Working:

• Rf of unknown = 4.8 / 8.0 = 0.60
• Rf of A = 2.4 / 8.0 = 0.30
• Rf of B = 4.8 / 8.0 = 0.60
• Rf of C = 6.0 / 8.0 = 0.75
• The unknown has the same Rf as dye B.

Answer: The unknown is dye B (Rf = 0.60)

Examiner note: Match by Rf value, not just by visual position. The Rf comparison is the scientific evidence.

Example 3: Multiple spots from a mixture

Question: A food colouring was separated by chromatography. The solvent front moved 12 cm. Three spots appeared at 3.6 cm, 7.2 cm, and 9.6 cm. Calculate the Rf value of each component.

Working:

• Spot 1: Rf = 3.6 / 12 = 0.30
• Spot 2: Rf = 7.2 / 12 = 0.60
• Spot 3: Rf = 9.6 / 12 = 0.80

Answer: Rf values are 0.30, 0.60, and 0.80

Examiner note: Each spot represents a different component of the mixture. Three spots means three components.

Example 4: Reverse calculation - finding distance

Question: A substance has an Rf value of 0.45. If the solvent front moved 12 cm, how far did the substance move?

Working:

• Distance = Rf x solvent front distance = 0.45 x 12 = 5.4 cm

Answer: 5.4 cm

Examiner note: The formula can be rearranged to find either distance when the other values are known.

Common mistakes

1. Measuring from the wrong point. Both distances must be measured from the baseline (the pencil start line), not from the bottom of the paper or from the spot itself.
2. Measuring to the edge of the spot instead of the centre. Always measure to the centre of the spot for accuracy.
3. Getting the fraction the wrong way round. It is substance distance divided by solvent front distance, not the other way round. If you get a value greater than 1, you have them backwards.
4. Forgetting that Rf depends on conditions. Rf values only match reference values when the same solvent, same paper, and same temperature are used. State this in comparison questions.

When this appears

Rf value questions appear on Paper 2 and Paper 4 and are accessible to both Core and Extended students. They connect to the experimental techniques topic and the practical skill of chromatography.