Experiment 7: Functional Groups & Thin Layer Chromatography

Question 1

Name three common uses of TLC

Answer 1

1. Used to identify compound and determine homogeneity of the sample
2. Monitor the progress of a reaction and verify the substance’s identity
3. Separate mixtures into its components and tell if it is pure

Question 2

Never touch the coated surface of the TLC plate with your fingers. Why?

Answer 2

• the oils and amino acids from our skin would go onto the plate and show up as spots and obstruct actual spots giving unreliable results

Question 3

Why should you not allow the solvent to migrate all the way to the end of the plate

Answer 3

• wouldn’t be able to see the spot
  = cannot measure distance travelled by spot
  = can’t calculate rf (retention factor)

Question 4

Why should you avoid pressing too hard while marking the coated surface of the plate?

Answer 4

• silica Gel would peel off plate

Question 5

What forces the mobile phase to move up the TLC plate?

Answer 5

The capillary action force

Question 6

Why is it not advisable to use ink to label your TLC plate?

Answer 6

• dye moves up plate as it develops => streaks ruin plate

Question 7

You are supposed to use the narrow end of the capillary tube to spot your sample on the TLC plate. Why?

Answer 7

• large spots would run into each other = ruin plate

- difficult to differentiate between samples

Question 8

Why should the beaker be covered during the development of the chromatograph?

Answer 8

So that the solvent moves up the plate instead of evaporated
- so that beaker becomes saturated with solvent vapor

Question 9

The solvent level in the developing beaker should always be below your TLC spots. Why?

Answer 9

• solvent would dissolve other samples

= results inaccurate

Question 10

The final chromatograph has fewer spots than expected and the ones that are visible are very faint. Why?

Answer 10

5 small spots were made on 1 area which then blended together to give us a single resultant spot for each sample.
The sample evaporated leaving only a faint mark.
The drops were too small and it was a dilute sample

Question 11

Your final chromatograph has spots that are dark enough to see, but they are very broad and overlap. Give reasons.

Answer 11

The drops were too large hence the samples mixed together

- the chromatograph was seen to have broad, overlapping spots

Question 12

There are no bands in the chromatograph, but the Solvent at the bottom of the beaker has a bright colour. What happened?

Answer 12

The sample solution became dissolved into the solvent and it colored the solution.
No bands formed