Interpreting Chromatograms Flashcards

All 8 cards in this deck

In paper chromatography, what is the mobile phase?

The solvent that moves through the paper.

In paper chromatography, what is the stationary phase?

The paper, which does not move.

Why do substances in a mixture end up at different heights on a chromatogram?

They move at different rates over the paper, depending on their attraction to the paper and solubility in the solvent.

On a chromatogram, how do you tell that a substance is pure?

It produces only one spot.

How is an unknown substance identified on a chromatogram using reference substances?

Its spot travels the same distance (same $R_f$) as the spot of a known reference substance run on the same paper.

What is the formula for the $R_f$ value?

$R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}}$

True or false? An $R_f$ value can be greater than 1.

False. The spot can never travel further than the solvent front, so $R_f$ is always between 0 and 1.

What are the steps to find the $R_f$ value of a spot on a chromatogram?

e.g. a spot 2 cm from the start line, solvent front 8 cm from the start line

1. Measure from the start (pencil) line to the centre of the spot: 2 cm.
2. Measure from the start line to the solvent front: 8 cm.
3. Divide: $R_f = 2 \div 8 = 0.25$