Chromatography (OCR GCE Chemistry A-Level)

'TLC' stands for

• Thick layer chromatography

• Thick liquid chromatography

• Thin liquid chromatography

• Thin layer chromatography

Answer explanation

The compounds move through a thin layer of silica or alumina

What is the plastic layer in a TLC plate?

• The mobile phase

• The stationary phase

• The solid support

• The solid sieve

Answer explanation

The stationery phase is stuck to it

What is SiO₂ commonly used for in TLC?

• The mobile phase

• The stationery phase

• The stationary phase

• The solid support

Answer explanation

The solvent flows over this stationary phase.

('Stationery' means envelopes / pens / pencils etc)

What would you need to carry out TLC? Tick all that apply

• Evaporating dish or cling film

• Beaker

• Solvent

• TLC plate

• Pencil

Answer explanation

You need the dish/cling film to stop the (harmful) solvent evaporating

The solvent in TLC is known as the

• Stationary phase

• Stationary layer

• Mobile layer

• Mobile phase

Through what process are different compounds separated during TLC?

Answer explanation

This is a very common question in OCR.

Learn the spelling!

Adsorption is the process through which solute can bond to the surface of the stationary phase

Which pigment is adsorbed least strongly?

• Yellow

• Blue

Answer explanation

The stronger the adsorption, the slower the compound will move and the lower the Rf

Which pigment is adsorbed most strongly?

• Blue

• Green

• Red

• Yellow

A chemist tries to separate a mixture of four aldehydes using TLC. What difficulty is she likely to have?

• Not many solvents will dissolve aldehydes

• All aldehydes are highly volatile - they'll evaporate

• They will adsorb too strongly to the stationary phase

• They will probably have very similar Rf values

Answer explanation

Compounds with the same functional group may well not separate through TLC: they'll be adsorbed the same amount as each other

A chemist wants to identify an unknown amino acid through TLC. Tick everything he needs to do

• Find a database of Rf values for all amino acids, using a readily-available solvent and stationary phase.

• Find a database of Rf values for all amino acids, using all possible solvents and stationary phases

• Run the TLC with the cheapest solvent and stationary phase

• Run the TLC with a solvent and stationary phase that matches the database

• Compare the Rf with the database to identify the amino acid

Answer explanation

He only needs reference Rf data for the solvent/stationary phase combination that he will use.