C5 Interpreting Chromatograms

Question 1

What can you expect to see on the chromatogram if the chemicals in the mixture are coloured?

Answer 1

A set of coloured dots at different heights on the chromatogram.

Question 2

What will you need to use if the chemicals in a mixture are colourless?

Answer 2

You will need to use locating agents to make them visible.

Question 3

If the mixture contains amino acids what locating agent can you use? And what will you see?

Answer 3

Ninhydrin solution.

If the mixture contains amino acids, you can spray the solution on the completed chromatogram, which should turn the spots purple.

Question 4

How can you use iodine crystals to see the chemicals on the paper of a chromatogram?

Answer 4

If you dip the completed chromatogram into a jar containing a few iodine crystals, the iodine vapour sticks to the chemicals on the paper and they’ll show up as purple spots.

Question 5

What is an Rf value?

Answer 5

The ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent.

Question 6

During a chromatography experiment, the solvent moves 4.8cm up the paper, and a solute moves 1.2cm. Calculate the Rf value of the solute.

Answer 6

0.25

Rf = distance travelled by solute = 1.2 =0.25
Distance travelled by solvent front 4.8

Question 7

What should Rf values always be less than?

Answer 7

1

If the answer is more than 1 check your calculations

Question 8

What is the formula to find Rf values?

Answer 8

Rf equals distance travelled by solute over distance travelled by solvent.

Question 9

What is chromatography often carried out to find out?

Answer 9

If a certain substance is present in a mixture.

Question 10

What would you need to do to check to see if a certain substance is present in a mixture using chromatography?

Answer 10

Run a pure sample of the substance alongside the unknown mixture.

If the Rf values match, the substances may be the same (although it doesn’t definitely prove they are)

Question 11

What can changes Rf values?

Answer 11

If the method used in the chromatography experiment changes

Question 12

List 4different methods that could change the Rf value.

Answer 12

If a different solvent is used
If a different concentration of solvent is used
The temperature is different
If a different stationary phase is used

Question 13

Why could it be useful to repeat the chromatography experiment with different conditions ?

Answer 13

If you get spots on a chromatogram that don’t separate properly, repeating the chromatography experiment with different conditions could cause them to separate.

Question 14

What may you need to do if two reference compounds have similar Rf values in a particular solvent?

Answer 14

Run the experiment again in conditions where the reference compounds have very different Rf values, so you can identify which ones in the mixture.

Question 15

The solvent on a chromatogram is at 6.0cm. There is one is able spot at 4.8cm

Calculate the Rf value of the visible spot. (2 marks)

Answer 15

Rf = distance travelled by solute = 4.8 (1 mark)
distance travelled by solvent front 6.0

= 0.80 (1 mark)

Question 16

Suggest how you could find out whether there are any colourless spots on the chromatogram (1 mark).

Answer 16

Spray the paper with ninhydrin solution/ put the paper in a jar containing a few iodine crystals (1 mark).