Required Practical: Investigating Chromatography Flashcards

All 5 cards in this deck

In the chromatography practical, why must the baseline be drawn in pencil and not in ink?

Pencil is insoluble in the solvent, so it will not dissolve and separate into spots of its own.

In the chromatography practical, why must the solvent level be below the baseline?

So the spots do not dissolve straight into the solvent in the container instead of travelling up the paper.

What are the steps to calculate an $R_f$ value from a chromatogram?

e.g. a spot has moved $3.0$ cm and the solvent front $12.0$ cm

1. Measure from the baseline to the centre of the spot: $3.0$ cm
2. Measure from the baseline to the solvent front: $12.0$ cm
3. Divide spot distance by solvent distance: $3.0 \div 12.0 = 0.25$ (2 s.f., no units)

Two spots on a chromatogram are very close together. Give two ways to increase the distance between them.

Allow the solvent front to travel further, and use a different solvent.

An ink contains four compounds but only two spots appear on the chromatogram. Suggest two reasons why.

Some of the compounds are colourless in solution, or some compounds have the same $R_f$ value so their spots overlap.