Question

A student sets up a paper chromatogram and places a spot of green food dye on the origin. After six minutes the solvent has moved 12 cm and a blue spot has advanced 9 cm. After fourteen minutes the solvent has advanced a further 8 cm. how many cm from the origin is the blue spot likely to be?
A. 15 cm
B. 6 cm
C. 18 cm
D. 26.36 cm

Answer 1

Hint: Paper chromatography is a technique used to separate mixtures of compounds into individual basings on their polarity. There are two phases involved in paper chromatography. One is a solid phase and another is a liquid phase.
Every molecule has its own ${{R}_{f}}$ value means retardation factor.
The formula to calculate retardation factor is as follows.
\[{{\text{R}}_{\text{f}}}\text{=}\dfrac{\text{distance travelled by the spot}}{\text{distance traveled by the solvent front}}\]

Complete step by step answer:
- In the question it is given that a green color spot is placed at origin. After 6 min the solvent has moved 12 cm (means solvent front) and a blue spot moved to 9 cm.
- Substitute the above data in the following equation
\[\begin{align}
 & {{\text{R}}_{\text{f}}}\text{=}\dfrac{\text{distance travelled by the spot}}{\text{distance traveled by the solvent front}} \\
 & \Rightarrow {{\text{R}}_{\text{f}}}\text{=}\dfrac{\text{9}}{\text{12}} \\
\end{align}\]

- Again after 14 min the solvent front advanced 8 cm more (means total solvent front is 20 cm) and we have to calculate after 14 min how much distance blue spot travelled.
- Assume the distance travelled by the blue spot as X. Then the retardation factor will be
\[\begin{align}
& \Rightarrow {{\text{R}}_{\text{f}}}\text{=}\dfrac{\text{distance travelled by the spot}}{\text{total distance traveled by the solvent front}} \\
& \Rightarrow {{\text{R}}_{\text{f}}}\text{=}\dfrac{\text{X}}{\text{20}} \\
\end{align}\]
- Therefore
\[\begin{align}
 & \Rightarrow \dfrac{9}{12}\text{=}\dfrac{\text{X}}{\text{20}} \\
 & \Rightarrow X=15cm \\
\end{align}\]
- Therefore the total distance travelled by the blue spot is 15 cm.
So, the correct answer is “Option A”.

Note: Based on the polarity of the solvent used in chromatography the distance travelled by the spots are going to change. If the solvent is highly polar then the spot covers a large distance on the paper chromatography.