Chromatography：练习题

In paper chromatography, why is the starting line for the samples drawn with a pencil instead of a ballpoint pen?

When setting up a paper chromatography experiment, a glass lid is often placed over the beaker containing the solvent and the chromatography paper.
What is the primary purpose of this lid?

Why is it important to stop the chromatography experiment before the solvent front reaches the very top edge of the paper?

In chromatography, why must the baseline (the starting line where the samples are placed) be drawn in pencil rather than in ink from a pen?

In paper chromatography, a locating agent is sometimes sprayed onto the paper after the solvent has finished moving.
Under which circumstance is a locating agent necessary?

When performing paper chromatography on a mixture of colorless substances, how can the positions of the spots be identified once the chromatogram is dry?

In a chromatography experiment, the baseline is \(2.0\ cm\) from the bottom of the paper. The solvent front reaches \(14.0\ cm\) from the bottom, and a component spot reaches \(11.0\ cm\) from the bottom. Calculate the \(R_f\) value for this component.

A sample of a food coloring is investigated using paper chromatography. The resulting chromatogram shows three distinct spots.

(a) State whether the food coloring is a pure substance or a mixture, and justify your answer.
(b) The solvent front travels a total distance of \( 15.0\text{ cm} \). One of the spots travels \( 6.0\text{ cm} \). Calculate the \( R_f \) value for this spot.
(c) If the experiment was repeated using a different solvent, explain why the \( R_f \) values of the components might change.

In an analytical laboratory, two chromatograms of the same unknown dye mixture are produced using two different solvents.

(a) In Solvent 1, the mixture separates into two spots. In Solvent 2, the same mixture separates into four spots. Deduce the minimum number of components in the mixture and explain your reasoning.
(b) Explain why a component might have an \( R_f \) value of \( 0.85 \) in one solvent but only \( 0.20 \) in another.
(c) Describe how chromatography can be used to monitor the progress of a chemical reaction.