All GRE Subject Test: Chemistry Resources
Example Question #1 : Thin Layer Chromatography
Which of the following functional groups would be expected to have the largest value during a thin-layer chromatography (TLC) experiment with an ether solvent?
The value is proportional to the affinity of the solute to the solvent. The solvent acts as the mobile phase along a polar paper stationary phase. Polar compounds will interact more with the paper, travelling slowly, while nonpolar compounds will interact more with the solvent, travelling more quickly.
A large value represents a large proclivity for the mobile solvent in the experiment. Because we are using ether, a non-polar solvent, we would expect non-polar compounds to travel the farthest on our plate. Of the answer choices, alkanes are the least polar and would thus travel farthest into the mobile phase of the four functional groups. A polar functional group, like a halide, will interact more in the stationary phase, and will thus has a significantly smaller value.
Example Question #2 : Thin Layer Chromatography
Which of the following purification techniques would best separate a nonpolar solute from a polar solute?
Thin layer chromatography
Ion exchange chromatography
Thin layer chromatography
Generally, extraction is the best means of separating two solutes based on polarity. This technique allows separation based on solubility in two different solvents, which separate based on polarity.
Extraction, however, is not offered as an answer. The next best option would be thin layer chromatography. In this process, a polar stationary phase is introduced to a nonpolar solvent. Solutes are placed on the stationary phase. The nonpolar solvent acts as the mobile phase. Nonpolar solvents interact more with the mobile solvent, travelling quickly along the polar stationary phase, while polar solutes are attracted to the stationary phase and travel more slowly. This property allows for separation based on polarity.
Ion exchange chromatography is used to separate compounds with different charges, not necessarily differing polarities. Mass spectroscopy will identify compounds based on mass, and distillation will allow for separation based on differences in boiling point and vapor pressure.
Example Question #3 : Thin Layer Chromatography
Chromatography involves the separation of a mixture by allowing a mobile phase to travel along a stationary phase. In thin layer chromatography (TLC), a liquid solution is able to travel along a stationary plate. The distance that a particular compound travels compared to another compound can be determined by comparing the Rf factors for each compound. The Rf factor is determined by dividing the compound's distance by the total distance of the solvent.
Which of the following compounds would have the smallest Rf factor in a standard thin-layer chromatography (TLC) experiment?
The stationary phase in chromatography is typically attracted to the more polar compounds in a solution, while the mobile phase carried the nonpolar compounds. As a result, more polar compounds will move a shorter distance, resulting in a lower Rf factor. Glucose is a very polar molecule, and would move a shorter distance compared to the other options.