Question

You are given a sample of several compounds to separate by paper chromatography. You draw a pencil line exactly 1.0 cm from the bottom of the paper, and place a spot of sample on it. You dry the sample, then develop it in a solvent.

When the chromatogram is taken out of the solvent, the paper is wet up to 9.1 cm from the bottom of the sheet. The compound you are interested in shows up as a spot 7.1 cm from the bottom of the paper.

Calculate the following:

In the same time, how far did the solvent move?

What is the Rf factor for the compund?

Answer 1

The spot of sample is made 1.0 cm away from the bottom of the paper.

Hence, that is the origin for all the distance calculation for solvent front and solute front.

Now, the paper is wet up to 9.1 cm from the bottom of the paper. Hence, solvent has moved 9.1-1.0 = 8.1 cm from the origin.

Hence, the distance solvent front = 8.1 cm.

The spot for the sample is found at 7.1 cm from the bottom of the paper. Hence, it is 7.1-1.0 =6.1 cm away from the origin.

Hence, the distance moved by the sample, i.e. solute front = 6.1 cm.

The situation can be visualized by the following diagram.

Now, we can answer the question as follows:

In the same time, how far did the solvent move?

The solvent moved from the origin to the solvent front i.e. 8.1 cm away.

Hence, the solvent has moved 8.1 cm.

What is the Rf factor for the compound?

Distance moved by the sample i.e. solute front = 6.1 cm

The R_f factor is determined as follows:

Hence, the R_f factor of the compound if 0.75 approximately.