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Difficulty: MediumPure and Impure Substances and Criteria of Purity

A quality control analyst evaluated a synthesized sample of benzoic acid using thin-layer chromatography. The solvent front advanced 10.0 cm10.0\text{ cm} from the origin line, while the compound produced a single distinct spot that moved 7.5 cm7.5\text{ cm}. What is the retardation factor (RfR_f) of the compound, and what does the result reveal about the purity of the sample?

  1. Rf=0.75R_f = 0.75, indicating that the sample is pure.Answer
  2. B
    Rf=1.33R_f = 1.33, indicating that the sample is pure.
  3. C
    Rf=0.75R_f = 0.75, indicating that the sample is impure because soluble impurities lower the boiling point.
  4. D
    Rf=1.33R_f = 1.33, indicating that the sample is an impure mixture.

Answer

The retardation factor is Rf=0.75R_f = 0.75, and the single chromatogram spot confirms that the sample is pure.
Calculating the retardation factor gives Rf=7.5 cm10.0 cm=0.75R_f = \frac{7.5\text{ cm}}{10.0\text{ cm}} = 0.75. In paper or thin-layer chromatography, pure substances yield a single spot under specified solvent conditions, confirming chemical purity.

Step-by-Step Solution

1
Calculate the retardation factor (RfR_f) using the chromatography ratio formula.
Rf=Distance moved by soluteDistance moved by solvent front=7.5 cm10.0 cm=0.75R_f = \frac{\text{Distance moved by solute}}{\text{Distance moved by solvent front}} = \frac{7.5\text{ cm}}{10.0\text{ cm}} = 0.75
The RfR_f value is the ratio of solute migration distance to solvent migration distance.
2
Interpret the number of spots on the chromatogram to assess purity.
A single distinct spot indicates only one component is present.
Impure samples produce multiple spots corresponding to different components in the mixture.

Key Concept

Criteria of Purity and Retardation Factor (RfR_f) Calculation
Estimated Time:1m 0s
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Topics

Pure and Impure Substances and Criteria of Purity