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?

15 cm

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6 cm

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18 cm

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26.6 cm

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Solution

The correct option is A 15 cm
For a particular solvent $R_{f}$ value remains constant and that $R_{f}$ is given by the formula
$R_{f} = \frac{distance advanced by the spot on TLC}{distance advanced by the solvent front}$ .
So here from the first date given we can find $R_{f} = \frac{9}{12}$
Let the distance moved by spot be x from origin.
So from the second set of data after 14 minutes , $R_{f} = \frac{x}{20}$
So, $\frac{9}{12} = \frac{x}{20} \Rightarrow x = 15 c m$
Answer is A

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