Purity, formulations and chromatography: Practice Questions

In paper chromatography, what is the purpose of the stationary phase?

Why might a single substance produce different \(R_f\) values in two different chromatography experiments?

Which of the following statements about chromatography is correct?

In chemistry, which of the following best defines a pure substance?

Which of the following products is an example of a formulation?

Explain how melting point data can be used to distinguish between a sample of pure water and a formulation such as sea water.

A student uses chromatography to analyze a medicine. They find that the medicine separates into four distinct spots on the chromatogram.

Is this medicine a pure substance or a formulation? Justify your answer using the principles of chromatography.

In a paper chromatography experiment, substance A moves 2.4 cm while the solvent moves 8.0 cm. Substance B moves 5.6 cm.

Calculate the \(R_f\) values for both substances and determine which substance has a higher affinity for the mobile phase.

A forensic scientist analyzes a liquid formulation suspected to be a mixture of food colorings.

a) Define the term formulation and explain how it differs from a pure substance in terms of its composition and purpose. (2)

b) Paper chromatography was used to separate the dyes in the formulation. The following results were recorded:
- Distance from start line to solvent front = \(12.5\text{ cm}\)
- Distance from start line to the center of Blue Dye spot = \(5.2\text{ cm}\)
- Distance from start line to the center of Red Dye spot = \(10.0\text{ cm}\)

i) Calculate the \(R_f\) value for the Red Dye. (2)
ii) The Blue Dye is known to have an \(R_f\) value of \(0.42\) in this specific solvent. Show by calculation whether the experimental result for the Blue Dye spot matches the known value. (2)

c) Explain why the start line must be drawn in pencil and not in ink. (1)