Question

A scientist investigates food colourings using paper chromatography.
Fig. 11.1 shows the chromatogram produced.
The result for dye A is not shown.

(a) Identify which dyes, B, C or D, are in the food colouring X.
……………………………………………………………………………………………………………………………
(b) The R$_{f}$  value of a food colouring is calculated using the formula 

    $R_{f}\tfrac{distance travelled by substance}{distance travelled by solvent}$

Calculate the R$_{f}$ value for dye B.

Show your working.

                                                        R$_{f}$ value = ………………………………………

(c) Food colouring A has an R$_{f}$ value of 0.44.

Calculate the distance travelled by food colouring A.

                                               distance travelled = ……………………………….. cm 

(d) The scientist makes a solution of food colouring B.
They dissolve 2.43g of the food colouring in 200cm$^{3}$ of distilled water.
Calculate the concentration of the solution made in mol/dm$^{3}$.
The relative molecular mass, M$_{r}$ , of the food colouring is 486.

                                        concentration = …………………………………… mol/dm$^{3}$

▶️Answer/Explanation

Ans:  11(a) B and C ; 
11(b) (R$_{f}$ =) 5.4 $\div $ 6(.0) ;
(R$_{f}$ =) 0.90 ;

11(c) (distance =) R$_{f}$ × distance moved by solvent / 0.44 × 6(.0) ;
(distance =) 2.64 or 2.6 (cm) ;

11(d) (moles =) 0.005 ;
(conversion of 200 cm$^{3}$

to dm$^{3}$ =) 0.2(00) (dm$^{3}$) ;
(concentration = 0.005 ÷ 0.200 =) 0.025 (mol / dm$^{3}$ ) ;

or
(conversion of 200 cm$^{3}$ to dm$^{3}$ =) 0.2(00) (dm$^{3}$) ;

(concentration =) 12.15g/dm$^{3}$) ;

(concentration in mol/dm$^{3}$= 12.15 ÷ 486 =) 0.025 (mol/dm$^{3}$) ;