Chromatography (rf Value)

Question

On a paper chromatogram, a coloured component travelled 2.7 cm from the baseline while the solvent front advanced 9.0 cm in the same run. Calculate the retardation factor of the component.

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Paper chromatography separates components by their differing affinities for the stationary phase, the water held by cellulose, and the moving mobile phase, the developing solvent. The retardation factor, R_f, is defined as the distance travelled by the component divided by the distance travelled by the solvent front, both measured from the baseline. Substituting the data, R_f = 2.7 / 9.0 = 0.30. Because both distances are lengths measured the same way, R_f is a dimensionless ratio that always lies between 0 and 1 for a component that moves slower than the solvent. Option 3.33 is the reciprocal, obtained by inverting the ratio, which is physically impossible since R_f cannot exceed 1. Option 0.70 is the complementary fraction representing how far the solvent moved beyond the spot, not the R_f itself. Option 0.27 comes from mistakenly dividing the spot distance by 10 instead of by the actual solvent distance. This definition matches the NCERT treatment of chromatographic separation. Plausibility check: a value of 0.30 lies correctly between 0 and 1, confirming the component lags behind the solvent front as expected.

Chromatography (rf Value)

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