Three organic compounds A, B and C were allowed to run in thin layer chromatography using hexane and gave the following result (see figure). The R_f value of the most polar compound is ____________ × 10⁻²

Solution

Chemistry ma’am · 3 steps

The most polar compound clings to the silica and travels the least, so look for the lowest spot.

1. Who moves least. On a TLC plate the stationary phase (silica gel) is polar and hexane is non-polar. A polar compound is held strongly by the silica, so it moves the shortest distance and has the smallest R_f.
2. Read the figure. The solvent front is 8 cm from the baseline. A has moved 6 cm, B 4 cm and C only 2 cm, so C is the most polar.

   R_f(C) = (distance moved by C)/(distance moved by solvent) = (2 cm)/(8 cm) = 0.25

3. So. R_f = 0.25 = 25 × 10⁻², so the answer is 25.

The common slip: 75. Taking the compound that moved farthest, A, gives 6/8 = 0.75, that is 75. A is the least polar compound, not the most polar.

The idea to keep. In TLC with a polar plate and a non-polar solvent, more polar means held more tightly, which means a smaller R_f. Always divide by the solvent front distance, not by the plate length.

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