A chloroform solution of pure organic compound was spotted at a distance \(0.80 cm\) from the base of a \(20 cm\) long chromatoplate. If the compound has the \(R _{ f }\) value of \(0.50\) and moves half way up the \(20 cm\) long plate, what is the distance of the solvent front from the top of the plate upon elution?
A. \(0.80 cm\) B. \(1.0 cm\) C. \(1.2 cm\) D. \(1.4 cm\)
Correct Answer: A
Explanation
Retention factor \(r_{ f }\) \(=\frac{\left(\begin{array}{l}\text { Distance travelled by } \\ \text { the solute from the origin }\end{array}\right)}{\left(\begin{array}{l}\text { Distance of solvent } \\ \text { front from origin }\end{array}\right)}\) \(0.5=\frac{10 cm -0.80 cm }{x-0.80 cm }\) \(0.5 x-0.40=10-0.80\) \(0.5 x=10-0.80+0.40=9.6\) \(0.5 x=9.6, x=\frac{9.6}{0.5}=19.2 cm\)istance of the solvent front from the top \begin{aligned} =20 cm -x &=20 cm -19.2 cm \\ &=0.8 cm \end{aligned}