Using the provided information in the following paper chromatogram :
Fig : Paper chromatography for compounds A and B.
the calculated R f value of A ________× 10⁻¹.

Solution

Chemistry ma’am · 4 steps

Measure both distances from the base line: spot A over the solvent front gives R_f straight away.

1. What Rf means. The retardation factor compares how far a compound travels with how far the solvent travels. Both distances are measured from the base line, where the spot was first placed.

   R_f = (distance moved by the compound from the base line)/(distance moved by the solvent front from the base line)

2. Read the chromatogram. Spot A sits 2 cm above the base line. The solvent front is 5 cm above the base line. The second 2 cm arrow runs from A up to B, so it belongs to B, not to A.
3. Put in numbers. R_f(A) = (2 cm)/(5 cm) = 0.4 = 4 × 10⁻¹. It has no unit because it is a ratio of two lengths.
4. So. The R_f value of A is 4 × 10⁻¹, so the answer is 4.

The common slip: 8. Adding both 2 cm arrows gives B's distance of 4 cm, so R_f = 4/5 = 0.8, which reads as 8. That is the R_f of B, not A.

The idea to keep. R_f is always compound distance over solvent distance, both from the base line, so it always lies between 0 and 1. Any value above 1 means you divided the wrong way.

Answer from the NTA final answer key. Solution by PrepWiser.