Chromatography: separating a mixture of inks | Exit Quiz | Oak National Academy

True or false? Rf values can be calculated from a chromatogram of unknown samples. These Rf values can be compared against sample data to identify unknown substances.

• true

• false

Answer explanation

Rf values can indeed be calculated for unknown samples and compared to those of known substances to help identify them. This is a common method in chromatography for substance identification.

Which of these does not need to remain constant for a fair comparison of Rf values between two chromatograms?

• the type of paper used

• the samples being analysed

• the size of the paper

• the temperature of the solvent

• the concentration of the solvent

Answer explanation

The size of the paper does not need to be the same for comparing Rf values, as Rf is a ratio of the distance travelled vs the solvent.

If a substance is insoluble in the chosen solvent, which of the following would happen?

• the substance will move with the solvent front

• the substance will have an Rf value above 1

• the substance will stay at the sample line

• the substance will have a negative Rf value

Answer explanation

If a substance is insoluble, it will not move from the sample line.

What is the Rf value of a solvent front that travels 8.0 cm and a substance spot that travels 20 mm?

Answer explanation

20 mm = 2 cm 2 ÷ 8 = 0.25 As both values in the question are to two significant figures (s.f.) the answer should be 0.25 (2 s.f.).

True or false? The temperature does not affect the outcome of the chromatography process.

• true

• false

Answer explanation

Temperature can affect the solubility of substances and therefore it can also affect their movement on the chromatography paper.

A chromatogram is collected and two different substances have similar Rf values. What might have caused this? They must have similar ___ in the solvent used.

Answer explanation

A chromatogram is collected, and two different substances have similar Rf values. This means that they must have similar solubilities in the solvent used.