Question 1
A spot moves 2.4 cm while the solvent front moves 6.0 cm. Calculate its Rf.
Need a hint for Question 1?
Divide the spot distance by the solvent-front distance.
Check Question 1
Rf = 2.4 ÷ 6.0 = 0.40. There is no unit.
STUDENT SPACE · TOPIC 5
Rf and colourless substances
Calculate Rf and explain how a locating agent reveals colourless substances.
The essentials
- Rf = distance travelled by the substance ÷ distance travelled by the solvent front, measured from the same baseline.
- Rf has no unit. Compare Rf values only under the same conditions.
- A locating agent can make colourless spots visible after the chromatogram has been developed and dried.
Key vocabulary
| English | Chinese |
|---|---|
| locating agent | 显色剂 |
| colourless | 无色的 |
| distance travelled | 移动距离 |
| Rf value | 比移值 |
Quick practice
Try each question before opening its hint or answer. Use paper for calculations and diagrams.
Question 2
A spot moves 3.6 cm and the solvent front moves 8.0 cm. Calculate Rf. In the same conditions, how far would it move if the solvent front moved 10.0 cm?
Need a hint for Question 2?
Use Rf = spot distance / solvent-front distance.
Check Question 2
Rf = 3.6 ÷ 8.0 = 0.45. Expected distance = 0.45 × 10.0 = 4.5 cm.
Question 3
Describe how to identify a colourless amino acid using paper chromatography and reference Rf values.
Need a hint for Question 3?
First reveal invisible spots, then compare under the same conditions.
Check Question 3
Develop the chromatogram in a suitable solvent. Mark the solvent front, dry the paper and apply a locating agent. Measure both distances from the baseline, calculate Rf and compare with reference values under the same conditions.
Use Lesson 5 in your worksheet for written practice. Your teacher will discuss those answers in class.