Question

A student developed a TLC plate by spotting the sample 1 cm from the bottom and allowed the solvent to move up to a point 1 cm from the top. He developed this TLC in 30% ethyl acetate in hexane and got a single spot with R_f value 0.3. If the distance travelled by the solvent front was 8 cm, calculate the distance travelled by the spot. Also, calculate the length of the TLC plate he used to do this experiment.

b.) does the experiment tell you about the polarity of the compound?

Answer 1

Spotting the sample 1 cm from the bottom and allowed the solvent to move up to a point 1 cm below top.

Total height moved by solvent is 8 cm, including bottom 1 cm. (see figure)

Thus, height of plate is 8 + 1 = 9 cm (1 cm not travelled at the top) (see figure)

Distance travelled by solvent is 7 cm above origin, spot travels by R_f = 30%.

100% is 7 cm, 30% is 2.1 cm. Thus, the spot travelled by 2.1 cm.

b. The compound is polar in btween alcohol and benzoic acid.