Three samples containing amino acids are analysed using paper chromatography. Sample 1 contains alanine only. Sample 2 contains glycine only Sample 3 contains equal amounts of two unknown amino acids. The resulting chromatogram is shown below. The Rf value for alanine is

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The question describes three samples analyzed by paper chromatography. Sample 1 contains alanine only, Sample 2 contains glycine only, and Sample 3 contains equal amounts of two unknown amino acids. The chromatogram is provided, and you are told the Rf value for alanine is 0.2 (as option c).

To evaluate Rf values in paper chromatography, use the formula: Rf = (distance traveled by the spot) / (distance traveled by the solvent front). The solvent front is the farthest distance reached by the solvent from the origin, and each compound’s spot distance is measured from the origin to where that compound’s spot stops.

Option c states Rf for alanine is 0.2. Consider why this makes sense given the setup:
- Since Sample 1 contains only alanine, the spot corresponding to alanine will appear at the position of alanine’s migration on the chromatogram. If the diagram indicates a spot at roughly one-fifth of the distance to the solvent front, that would correspond to an Rf ≈ 0.2.
- Alanine is less polar than glycine (glycine is smaller and more polar due to having only a hydrogen side chain), so glycine typically migrates differently, often with a lower or somewhat different Rf depending on solvent system. This helps distinguish alanine’s position from glycine’s spot when comparing Sample 2 to Sample 1.
- For Sample 3, the two unknown amino acids will produce two distinct spots; their relative positions can be compared to the known alanine spot (from Sample 1) to help identify them, but that identification isn’t required to determine alanine’s Rf value in Sample 1.

If there were other options, one would evaluate them by whether their Rf values align with the measured distance of alanine’s spot relative to the solvent front. An Rf significantly smaller than 0.2 would place alanine much closer to the origin, and a value much larger than 0.2 would place it closer to the solvent front, neither of which would match the observed spot for alanine in Sample 1.

In summary, the stated Rf value of 0.2 for alanine is reasonable if the location of alanine’s spot on the chromatogram is about one-fifth of the distance to the solvent front, consistent with typical behavior of alanine under common paper chromatography conditions.

Note: With the provided information, we are treating the sole alanine spot from Sample 1 as the reference for alanine’s Rf, and the rest of the interpretation (glycine’s spot and the two unknowns in Sample 3) serves to contextualize why 0.2 is a plausible Rf for alanine in this setup.

Three samples containing amino acids are analysed using paper chromatography. Sample 1 contains alanine only. Sample 2 contains glycine only Sample 3 contains equal amounts of two unknown amino acids. The resulting chromatogram is shown below. The Rf value for alanine is单项选择题

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