A student is designing an experiment using paper chromatography. From previous results, she knows that the Rf values for two dyes that she wishes to investigate are 0.72 and 0.80.

It is proposed that the dyes be run on the same piece of paper and, for various reasons, that the sports should have a separation of a at least 1cm at the end of the procedure.

What is the minimum height of the chromatography paper that she should use in this experiment?

Can someone correct my working out. I don't know what the correct answer is.

  • distance travelled by dye x = $x$
  • distance travelled by dye y = $y$
  • distance of solvent front = $p$
  • $x/p=0.72$ therefore $x = p(0.72)$
  • $y/p=0.8$, therefore $y = p(0.8)$
  • $y-x=1$, therefore $y=1+x$
  • new equation: $1+x=p(0.8)$ where $x=0(0.72)$, if we solve for $p$ we get $12.5$

so the piece of paper should be at-least 12.5 cm long.


1 Answer 1


$$\Delta R_f = 0.08$$ Let the separation distance be $\Delta r$ = 1 cm and let $l$ be the length of the paper in cm.

$$\Delta R_f = \frac{\Delta r\, \mathrm{cm}}{l\, \mathrm{cm}} $$

Solve for $l$, obtain $l$ = 12.5 cm.

  • $\begingroup$ why is the change in rf over 1 $\endgroup$
    – confused
    Commented Apr 18, 2014 at 9:16

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