Snape grows tired of these conceptual questions and thinks it's time for a problem. What is the retention factor if the distance traveled by the solvent front is 2.00 cm, and the distance traveled by the ion is 0.40 cm?

ion travelled : 2.00cm - 0.40cm

= 1.6cm

∴ Rf = 1.6/2.0

= 0.80

Explanation:

Retention factor is the ratio of distance travelled by solute divided by distance travelled by solvent.

since you given distance travelled by solvent then the distance required by solute is need in this case the ion.