A student sets up a paper chromatography experiment and places a spot of food dye on the origin. They remove the paper after 6 minutes and the solvent has moved 4.0 cm and a blue spot has moved 3.0 cm. The student runs the experiment again under identical conditions, however they leave experiment to run for 12 minutes. The solvent moved this time moved 6.0 cm. What distance did the blue spot move in this experiment?

Question

BlackTom AI · Accepted Answer

A key idea in paper chromatography is that the relative position of a compound to the solvent front is described by the Rf value, which remains constant under identical conditions. In the first run, the solvent front moved 4.0 cm and the blue dye moved 3.0 cm, giving an Rf = 3.0 / 4.0 = 0.75. If the second run is done under the same conditions, the dye should travel a distance proportional to the solvent front, so the expected blue spot distance is 0.75 × 6.0 cm = 4.5 cm.

Option a. 4.5 cm: This aligns with the constant Rf value method, making it the mathematically consistent choice given the data and assumptions.

Option b. 1.5 cm: This would correspond to a much smaller fraction of the solvent front than observed initially (1.5/6.0 = 0.25), which contradicts the same-Rf principle under identical conditions.

Option c. 3.0 cm: While this matches the initial distance in the first run, it does not scale with the increased solvent front distance in the second run, violating the idea that the dye’s movement is proportional to the solvent front under consistent conditions.

Option d. 6.0 cm: This would indicate the dye moved as far as the solvent front itself, which would imply an Rf of 1.0. Since the initial Rf was 0.75, this would be inconsistent with the observed relationship between solvent front and dye movement in the first run.

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