Purity, Formulations & Chromatography Practice Questions

State what is meant by a pure substance in chemistry.

The spring water contains dissolved minerals.

Explain why the spring water is not pure in the chemical sense.

Which substance is pure?

Tick (✓) one box.

• Air
• Brass
• Distilled water
• Sea water

What is meant by a formulation?

Which two of these are formulations?

Tick (✓) two boxes.

• Brass
• Chlorine
• Distilled water
• Iron
• Paint

A paint manufacturer measures the quantity of each component very carefully.

Give one reason why.

What is the mobile phase in paper chromatography?

Tick (✓) one box.

• Beaker
• Lid
• Paper
• Solvent

Write the equation used to calculate an $R_f$ value.

A sample of a yellow dye produces one spot on the chromatogram in three different solvents.

What does this tell you about the dye?

Which sample is pure aspirin?

Give a reason for your answer.

What does the result for sample A tell you about sample A?

Suggest which sample contains the greatest amount of impurity. Justify your answer using the data.

Calculate the $R_f$ value of pigment X.

$R_f$ value $= \rule{2cm}{0.15mm}$

The student then ran a chromatogram of a different pigment, Z, using the same solvent. The solvent again moved 8.0 cm from the start line.

The $R_f$ value of pigment Z is $0.15$

Calculate the distance moved by pigment Z from the start line.

Distance moved $= \rule{2cm}{0.15mm}$ cm

The student drew the start line in pencil.

Give one reason why the start line must not be drawn in ink.

Explain why the level of the solvent in the beaker must be below the start line.

Explain how paper chromatography separates the dyes in the sports drink.

How does the chromatogram show that the coating is not a pure substance?

Calculate the $R_f$ value of each of the two spots.

Use the table and your answers to part b to identify the two dyes in the sweet coating.

Explain why this fertiliser is a formulation.

Suggest one reason why the manufacturer must measure the mass of each component accurately.

Give one example of a formulation other than a fertiliser.

Calculate the $R_f$ value of the dye in solvent A.

Give your answer to 2 significant figures.

$R_f$ value $= \rule{2cm}{0.15mm}$

The experiment was repeated with the same dye and the same paper, but using solvent B.

The $R_f$ value of the dye in solvent B was 0.62

Explain why the same dye can have different $R_f$ values in different solvents.

A second dye has an $R_f$ value of 0.95 in solvent B.

Suggest what this tells you about the attraction of this dye to the paper.

The student drew the start line using one of the purple markers.

Explain how this would affect the chromatogram produced.

In a second attempt, all of the spots from all three markers travelled to the solvent front, giving $R_f$ values of 1.0

Suggest why this happened and suggest one improvement the student could make.

Using a third solvent, the ink from one marker produced a single spot, but with a fourth solvent the same ink produced two spots.

Explain what this shows about the ink and why the number of spots was different in the two solvents.