Question

10. On the TLC plate below, identify the spots in the unknown mixture as compound A, B, and C and calculate all Rt values. Lanes with X are unknown sample spots. State whether the solvent system used is a great one for the separation of the compounds. + A B X

Answer 1

R_f value calculation:

R_f value is the ratio of the distance travelled by solute from the origin to the distance travelled by the solvent from the origin. It is expressed as,

R_f = Distance travelled by solute / Distance travelled by solvent

Distance travelled by solute (compound) A, B, C and solvent from the origin are measured and given below.

Distance travelled by compound A (D_A) = 2.5 cm

Distance travelled by compound B (D_B) = 5.75 cm

Distance travelled by compound C (D_C) = 2.6 cm

Distance travelled by solvent  (D_S) = 8.3 cm

R_f(A) = D_A / D_S = 2.5 cm / 8.3 cm = 0.30

R_f(B) = D_B / D_S = 5.75 cm / 8.3 cm = 0.69

R_f(C) = D_C / D_S = 2.6 cm / 8.3 cm = 0.31

Compound	Rf Value
A	0.30
B	0.69
C	0.31

Identification of compound A, compound B & compound C:

In the TLC, spot at R_fvalue 0.30 is compound A, spot at R_fvalue 0.69 is compound B & spot at R_fvalue 0.31 is compound C.

Summary:

Compound B is well separated from compound A & compound C. Compound A & C are having very close R_f value. Hence the mobile phase used in the TLC is not suitable for the separation of compound A & compound C.

Bo Bo Bo - - - - А . А. .-. -- Do pe Da Do - - + - - - - . - - LORIGIN A + * B C DA= 2.5 DB = 5.75 Do = 2.6 Ds: 8.3