Q: Which of the following is the correct formula for calculating the Rf value in chromatography?

$$Rf = \frac{\text{distance traveled by compound}}{\text{distance traveled by solvent}}$$

Question 1

In thin-layer chromatography (TLC), what is the primary reason that different compounds travel different distances up the plate?

Accepted Answer

Because of differences in their affinity for the stationary and mobile phases.

Question 2

In a TLC experiment using a polar stationary phase (silica gel) and a nonpolar mobile phase, which type of compound will travel the shortest distance up the plate?

Accepted Answer

A polar compound

Question 3

Given the following Rf values from a TLC plate: red = 0.50, green = 0.75, which compound is more strongly attracted to the stationary phase?

Accepted Answer

The red compound

Question 4

A student runs a TLC experiment and measures the distance traveled by the solvent front as 8.0 cm. A blue spot travels 2.0 cm from the origin. What is the Rf value for the blue compound?

Accepted Answer

$$Rf = \frac{2.0}{8.0} = 0.25$$

Question 5

Which of the following best describes the role of the mobile phase in chromatography?

Accepted Answer

It moves up the plate, carrying the components of the mixture with it.

Question 6

If a compound has a very high affinity for the mobile phase and a low affinity for the stationary phase, what would you expect its Rf value to be?

Accepted Answer

Close to 1

Question 7

A TLC plate is run with a polar stationary phase and a nonpolar solvent. Which of the following changes would increase the Rf values of all compounds?

Accepted Answer

Using a more polar solvent

Question 8

A student accidentally allows the solvent front to run off the top of the TLC plate. How does this affect the calculation of Rf values?

Accepted Answer

Rf values cannot be calculated because the distance traveled by the solvent is unknown.

Question 9

Which of the following is the correct formula for calculating the Rf value in chromatography?

Accepted Answer

$$Rf = \frac{	ext{distance traveled by compound}}{	ext{distance traveled by solvent}}$$

Question 10

In a TLC experiment, a student observes that two compounds have identical Rf values. What is the best conclusion?

Accepted Answer

The two compounds have similar affinities for the stationary and mobile phases under these conditions.

Question 11

A mixture is separated by TLC using a silica plate and a nonpolar solvent. The following Rf values are obtained: compound A = 0.10, compound B = 0.60. Which compound is more likely to be nonpolar?

Accepted Answer

Compound B

Question 12

Why is it important to keep the origin (where the sample is spotted) above the level of the solvent in the developing chamber during TLC?

Accepted Answer

To prevent the sample from dissolving directly into the solvent reservoir.

Question 13

A TLC plate is developed and the solvent front travels 5.0 cm. A green spot travels 3.75 cm. What is the Rf value for the green compound?

Accepted Answer

$$Rf = \frac{3.75}{5.0} = 0.75$$

Question 14

Which of the following statements about the stationary phase in TLC is correct?

Accepted Answer

It is typically a solid, such as silica gel, that remains fixed on the plate.

Question 15

A TLC plate is run with a mixture of three compounds. The Rf values are 0.20, 0.50, and 0.80. Which compound is most likely to be the least polar?

Accepted Answer

The compound with Rf = 0.80

Question 16

In a real-world scenario, a forensic chemist uses TLC to analyze ink from a crime scene. The ink sample produces two spots with Rf values matching those of a suspect's pen. What is the best interpretation?

Accepted Answer

The ink from the crime scene and the suspect's pen are likely to contain the same compounds.

Chromatography: Principles and Applications in GOB Chemistry

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