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Difficulty: MediumPure and Impure Substances and Criteria of Purity

A student performed a paper chromatography experiment to analyze the purity of an organic dye sample. On the chromatogram, the solvent front traveled 8.0 cm8.0\text{ cm} from the baseline, and a single distinct spot traveled 5.0 cm5.0\text{ cm}. Which of the following gives the correct RfR_f value and the accurate deduction about the purity of the dye sample?

  1. Rf=0.625R_f = 0.625, indicating that the sample is a pure substance.Answer
  2. B
    Rf=1.60R_f = 1.60, indicating that the sample is a pure substance.
  3. C
    Rf=0.625R_f = 0.625, indicating that the sample is a mixture of several components.
  4. D
    Rf=1.60R_f = 1.60, indicating that the sample is an impure mixture.

Answer

The retention factor is Rf=0.625R_f = 0.625, which indicates that the dye sample is a pure substance because only a single spot appeared on the chromatogram.
The retention factor (RfR_f) is determined by dividing the distance moved by the solute spot (5.0 cm5.0\text{ cm}) by the distance moved by the solvent front (8.0 cm8.0\text{ cm}), giving Rf=0.625R_f = 0.625. Furthermore, a single spot on the paper chromatogram serves as a standard criterion of chemical purity, confirming that the sample is a pure substance.

Step-by-Step Solution

1
Calculate the retention factor (RfR_f) using the formula Rf=Distance moved by soluteDistance moved by solvent frontR_f = \frac{\text{Distance moved by solute}}{\text{Distance moved by solvent front}}.
Rf=5.0 cm8.0 cm=0.625R_f = \frac{5.0\text{ cm}}{8.0\text{ cm}} = 0.625
The RfR_f value represents the relative distance traveled by the compound compared to the solvent front.
2
Interpret the number of spots observed on the developed chromatogram.
A single spot confirms the presence of only one chemical component.
Pure substances produce a single spot under specified solvent conditions, whereas mixtures resolve into two or more distinct spots.

Key Concept

Criteria of purity in chromatography: pure substances yield a single spot with a characteristic RfR_f value between 0 and 1.
Estimated Time:1m 0s
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Topics

Pure and Impure Substances and Criteria of Purity