Required practical 6 - Chromatography Flashcards
What is an Rf value?
A Rf value is the ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent.
How can you use chromatography to see if a certain substance is present in a mixture?
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.
How do you calculate Rf values?
Rf = distance travelled by substance / distance travelled by solvent
How could you use paper chromatography to separate a mixture of food colourings?
- Draw a pencil line 2cm 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 1cm 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.
Why should pencil be used to draw the line along the bottom of the chromatography paper?
It will not affect the experiment as it is insoluble in the solvent.
Why should the water in the beaker for paper chromatography be no deeper than 1cm deep?
If it is deeper, it will wash away the substances placed on the line on the chromatography paper.
In paper chromatography, what is the stationary phase?
The chromatography paper.
In paper chromatography, what is the mobile phase?
The solvent.
What 2 things affect how long the molecules spend in each phase in paper chromatography?
- How soluble they are.
- How much they are attracted to the chromatography paper.
What is the Rf value dependent on?
The Rf value is dependent on the solvent.
If you repeat the experiment with a different solvent, the Rf value will change.
How does paper chromatography work to separate a mixture?
- 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.
When measuring the distance moved by a substance on the chromatography paper, where should you measure between?
Measure from the pencil baseline to the spot of substance.
Why should you use a lid when carrying out paper chromatography?
To prevent the solvent evaporating.
