Required Practical 6 - Chromatography

Question 1

What is an Rf value?

Answer 1

A Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent

Question 2

How can you use chromatography to see if a certain substance is present in a mixture?

Answer 2

• Run a pure sample of this substance alongside the unknown mixture.
• If the Rf value of the pure substance matches the value of one of the spots from the mixture, it is
  likely to be present.

Question 3

How do you calculate Rf values?

Answer 3

Rf = Distance travelled by substance / Distance travelled by solvent

Question 4

How could you use paper chromatography to separate a mixture of food colourings?

Answer 4

• Draw a pencil line 2 cm from the bottom of chromatography paper
• Mark 4 pencil spots along this horizontal line
• On each pencil mark, place one dot of 3 known food colourings and one dot of an unknown mixture, label each dot
• Place the chromatography paper in a beaker containing 1 cm of water
• Wait for the solvent (water) to travel up most of the paper
• Remove from the beaker and draw a line where the solvent reached
• Hang the paper up to dry
• Calculate the Rf values

Question 5

Why should pencil be used to draw the line along the bottom of the chromatography paper?

Answer 5

It will not affect the experiment as it is insoluble in the solvent

Question 6

Why should the water in the beaker for paper chromatography be no deeper than 1 cm deep?

Answer 6

If it is deeper it will wash away the substances placed on the line on the chromatography paper

Question 7

In paper chromatography, what is the stationary phase?

Answer 7

The chromatography paper

Question 8

In paper chromatography, what is the mobile phase?

Answer 8

The solvent, for e.g. water or ethanol

Question 9

What two things affect how long the molecules spend in each phase in paper chromatography?

Answer 9

• How soluble they are
• How much they are attracted to the chromatography paper

Question 10

What is the Rf value dependent on?

Answer 10

• The Rf value is dependent on the solvent.
• If you repeat the experiment with a different solvent, the Rf value will change.

Question 11

How does paper chromatography separate a mixture?

Answer 11

• The mobile phase moves through the stationary phase so anything dissolved in it will move with it
• Compounds interact differently with each phase so will move different amounts through the stationary phase and so will separate

Question 12

When measuring the distance moved by a substance on the chromatography paper, where should you measure between?

Answer 12

Measure from the pencil baseline to the middle of the spot of the substance

Question 13

Why should you use a lid when carrying out paper chromatography?

Answer 13

To prevent the solvent evaporating