CHE2304: QUIZ 1-B

Which of the following statements about paper Rf and column chromatography Rt is correct?

A long retention time in gas chromatography is indicative of a substance with a strong adsorption on to the stationary phase.

Oil and water may be separated by using

A separating funnel

Why is it difficult to extract the EDTA complex of aluminum into an organic solvent but easy to extract the 8-hydroxyquinoline complex? (2 marks)

Aluminum cation has oxidation state of +3, while EDTA anion is -4. The EDTA complex formed therefore has an overall -1 charge. This will remain soluble in aqueous phase. The 8-hydroxyquinoline complex is neutral since its made up of one Al cation and 3 8- hydroxyquinoline molecules. This will make it soluble in organic phase.

Which statement about TLC is correct?

Compounds that dissolve rapidly in the chosen eluent move with the solvent front.

Solvent passes through a column in 3.0 min but solute requires 9.0 min. b) What fraction of time is the solute in the mobile phase in the column? (2 Marks)

Fraction of time solute is in mobile phase = tm/tr = 3.0 min /9.0 min = 0.33

If you wish to extract aqueous benzylamine into hexane, is it more effective to adjust the aqueous phase to pH 3 or pH 8? Explain. (2 Marks)

It is more effective to adjust the aqueous phase to pH 8. At this pH, benzylamine will remain in its neutral form and there will be extracted into the organic phase.

If you need to bring the EDTA complex into the organic solvent, should you add a phase transfer agent with a hydrophobic cation or a hydrophobic anion? (2 Marks)

Since the EDTA complex has a negative charge, you'll need to add a hydrophobic cation to bring the resulting complex into the organic phase.

In chromatography, the stationary phase can be ___________ supported on a solid.

Solid or liquid

After performing a TLC experiment, a researcher determines the Rf value of a component to be 2. He also notes that the solvent travelled a distance of 4cm on the plate. What can you conclude about this experiment?

The reported data does not seem valid

In extraction, the effectiveness of a solvent can be measured by the

a) Distribution coefficients b) Selectivity c) *Both (a) and (b)* d) Diffusivity

What are the important properties of a good solvent for extraction processes?

a) The chosen solvent should be highly selective for the solute to allow high solubility. b) Since a lot of solvent is needed, it should be as cheap as possible. c) The chosen solvent should have a lower selectivity for the solute to allow high Separation *d) a and b

The diagram shows the apparatus for separating soil and water. What are the labelled parts?

d) A = residue, B = filtrate

Solvent passes through a column in 3.0 min but solute requires 9.0 min. (a) Calculate the retention factor, k. (2 Marks)

k = (tr - tm)/tm = (9.0 - 3.0) min/ 3.0 min = 2.0

Thin layer chromatography can be used to distinguish between different amino acids. If a particular amino acid has low solubility in the mobile phase used then the amino acid

will have a low Rf value.