Paper chromatography
3 marks · EasyThe chromatogram below was obtained by separating a mixture of food dyes A and B. The solvent front travelled 6.0 cm from the pencil line. The diagram shows the position of the spots:
(a) State how the R_f value for dye B can be calculated. [1]
(b) Calculate the R_f value for dye B. Give your answer to two decimal places. [2]
State how the R_f value for dye B can be calculated.
Calculate the R_f value for dye B. Give your answer to two decimal places.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) By dividing the distance moved by the dye by the distance moved by the solvent front from the pencil line.
(b) 0.80
Common mistakes: Dividing solvent front by spot distance; Incorrect measurement
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that R_f is calculated as (distance moved by spot)/(distance moved by solvent front).
M1 (b) · 1 mark · Measures/calculates the distance moved by dye B as 4.8 cm and the solvent front as 6.0 cm; sets up correct ratio.
A1 (b) · 1 mark · Calculates the correct R_f value for dye B as 0.80 (to two decimal places). (follow-through)
Worked solution
a) The R_f value is calculated by dividing the distance moved by the spot by the distance moved by the solvent front, both measured from the pencil line.
b) Distance moved by dye B = 4.8 cm Distance moved by solvent front = 6.0 cm