Chromatography Flashcards

All 7 cards in this deck

In paper chromatography, what is the stationary phase and what is the mobile phase?

Stationary phase: the paper. Mobile phase: the solvent.

In paper chromatography, why does one dye in a mixture travel further up the paper than another?

It is more soluble in the solvent and has a weaker attraction to the paper, so it spends more time in the mobile phase.

What does separation in chromatography depend on?

The distribution of the substances between the stationary phase and the mobile phase.

What is the formula for the $R_f$ value of a spot?

$R_f = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent (front)}}$ — it has no units.

What are the steps to find the distance moved by the solvent front from an $R_f$ value?

e.g. $R_f = 0.50$ and the dye has moved $4.0$ cm

1. Substitute into $R_f = \dfrac{\text{distance moved by dye}}{\text{distance moved by solvent}}$: $0.50 = \dfrac{4.0}{d}$
2. Rearrange: $d = \dfrac{4.0}{0.50}$
3. $d = 8.0$ cm

Why can the same substance have a different $R_f$ value in a different solvent?

Because its solubility in the new solvent is different, so it moves a different distance relative to the solvent front.

True or false? A pure compound produces a single spot in all solvents.

True. A mixture may separate into two or more spots, and the number of spots can change with the solvent.