Two compounds I and II are eluted by column chromatography (adsorption of I > II). Which one of the following is a correct statement?

We are told that, in column chromatography, compound I is adsorbed more strongly than compound II, i.e. $$\text{adsorption of I} > \text{adsorption of II}.$$

In column chromatography the stationary phase is an adsorbent, often silica or alumina. A greater degree of adsorption means a stronger interaction with the stationary phase, so the compound travels down the column more slowly. Conversely, a compound that is adsorbed less strongly interacts weakly with the stationary phase and is carried downward more quickly by the mobile phase.

Therefore, because I is more strongly adsorbed, I moves slower through the column, while II, being less strongly adsorbed, moves faster.

Now let us connect this to the $$R_f$$ value, which is defined (for thin-layer chromatography, but the qualitative trend is the same) as

$$R_f = \dfrac{\text{distance travelled by the solute spot}}{\text{distance travelled by the solvent front}}.$$

If a compound travels a larger distance on the plate (or equivalently elutes faster in a column), the numerator in the above fraction is larger, giving a higher $$R_f$$. Thus, the compound that moves faster automatically acquires the higher $$R_f$$ value.

Since we have already established that II moves faster than I, it must also have the higher $$R_f$$ value.

Putting the two conclusions together:

• II moves faster than I, and
• II has the higher $$R_f$$ value.

This statement matches exactly with Option B.

Hence, the correct answer is Option B.