Question

How is the retention factor or ${R_f}$ of a particular compound measured in a thin layer of chromatography (TLC) experiment?
A.Distance travelled by the compound on the TLC plate divided by the total distance travelled by the mobile phase.
B.Distance travelled by the compound on the TLC plate multiplied by the total distance travelled by the mobile phase.
C.The total distance travelled by the mobile phase divided by the distance travelled by the compound on the TLC plate.
D.The total distance traveled by the mobile phase multiplied by the distance travelled by the compound on the TLC plate.

Answer 1

Chromatography is a method of separation of mixtures into its components, for purification of compounds and also for testing the purity of compounds. It is based on the principle of the difference in the rate at which the components of mixture move through a porous medium under the influence of a solvent or gas.

Complete step by step answer:
Chromatography has two phases- one is the stationary phase of a large surface area and the second is the moving phase which moves slowly over the stationary phase.
In Thin layer of chromatography, the substances of a mixture are separated over a thin layer of an absorbent coated over a plate. This method involves following steps-
A thin layer of absorbent material like silica gel or alumina is spread over a glass plate as the stationary phase. This plate is called a chroma plate.
The solution of mixture which is to be separated is applied as a small spot with help of a fine capillary above one end of the TLC plate.
The spot is dried and is placed in a closed jar containing the solvent covered with a lid. The solvent moves up the plate and the components of mixture move up along the plate to different distances depending on the degree of their adsorption.
The movement of substance relative to the solvent is expressed in terms of retention factor and is given as-
${R_f} = \dfrac{{{\text{Distance moved by substance or compound}}}}{{{\text{The total distance travelled by solvent(mobile phase)}}}}$
This factor is constant for a given substance under a given set of conditions.

The correct answer is option A.

Note:
The retention factor helps in-
Identifying the various components.
Estimating the components quantitatively by measuring the intensity of colors developed by them on reacting with suitable reagent.