CHROMATOGRAPHY

Chromatography usually involves two different phases. What names are given to these two phases?

• Mobile and stationary

• Soluble and insoluble

• Solvent and solid

• Wet and dry

• Positive and negative

Why are chromatograms spotted with amino acid solutions often sprayed with ninhydrin after the chromatography is complete?

• To make the spots visible on the paper

• To stop the spots from moving further up the paper

• To remove any impurities

• To preserve the chromatogram for several years

• To make the chromatogram safe to handle

A chromatography experiment was performed on five different solutions of amino acids (A–E). A contained cysteine, B contained glycine, C contained alanine, D contained valine, and E contained an unknown mixture of amino acids. By inspecting the chromatogram shown, which of the amino acids are present in solution E?

• Glycine and valine

• Cysteine and alanine

• Alanine and valine

• Cysteine and valine

• Glycine and alanine

Shown below is a chromatogram of three unknown amino acids, A, B, and C, and a reference table of 𝑅f values of various amino acids. What are the three amino acids?

• A: proline, B: lysine, C: glycine

• A: glycine, B: alanine, C: proline

• A: lysine, B: glycine, C: leucine

• A: alanine, B: proline, C: lysine

• A: leucine, B: alanine, C: lysine

A student is running a chromatography experiment. The diagram below shows their setup at the start of the experiment. What mistake has the student made?

• The level of solvent is above the baseline.

• The paper has not been sprayed with ninhydrin.

• A lid has been placed over the beaker.

• There is no thermometer to measure the temperature of the solvent.

• The spots are not below the baseline.

The given chromatogram shows that substance B has traveled further up the chromatography paper than A. Which properties of a substance affect how far it will travel up the chromatography paper?

• Its molecular weight and density

• Its position in the reactivity series and reaction with water

• Its boiling point and volatility

• Its solubility in the solvent and attraction to the paper

• Its color and pH

The 𝑅f value for valine is 0.60. If the solvent line was measured to have moved 4.0 cm from the starting point, how far from the starting point will a spot of valine have moved?

In a paper chromatography experiment, a student placed an ink mixture in a solvent beaker covered with a lid. He observed that the ink is not rising at all. Which of the following is the problem he encountered?

• The mobile phase is not evaporating.

• The ink is insoluble in the mobile phase.

• The ink has high affinity with the mobile phase.

• The mobile phase is evaporating.

• The ink is soluble in the mobile phase.

A student is required to conduct a paper chromatography experiment to separate an unknown mixture. On a filter paper, he spotted two drops of the mixture on a line he drew using a pen. After that, he placed the paper on a solvent beaker and put a lid above the beaker. When his work was evaluated, he had points deducted because the results were not in the correct range. What mistake did he make?

• Using a pen to draw the line

• Using filter paper

• Putting a lid on the beaker

• Using a beaker

• Adding two drops of the mixture

In TLC experiments, why is it important to place a lid over the solvent beaker?

• To prevent air from entering the beaker

• To preserve the chromatogram for several months

• To protect the system from light

• To prevent mobile phase evaporation

• To prevent stationary phase evaporation