C8bD Chromatography and Instrumental Analysis: Exam Drill

AQA 8464 · Calculator allowed · about 30 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Paper chromatography separates dyes using a solvent moving up a strip of chromatography paper.

(a)Define Rf value.(1)

(b)Give the equation used to calculate Rf.(1)

(Total for Question 1 is 2 marks)

A student runs paper chromatography. The solvent front moved 84 mm from the origin. A blue spot moved 42 mm from the origin.

(a)Calculate the Rf value for the blue spot.(2)

(Total for Question 2 is 2 marks)

In paper chromatography, the solvent sometimes passes the origin line and dissolves the spots, causing streaking.

(a)Give two practical changes to reduce streaking.(2)

(Total for Question 3 is 2 marks)

Thin layer chromatography (TLC) is similar to paper chromatography but uses a thin coated plate and can separate more small organic molecules.

(a)State one advantage of TLC over paper chromatography and explain why.(3)

(Total for Question 4 is 3 marks)

A required practical asks students to identify components in a mixture using paper chromatography of inks. The origin is 10 mm from the bottom. The solvent front reached 70 mm. A red spot from the mixture is at 40 mm and a red spot from a known dye is at 42 mm.

(a)Decide whether the known dye is present and justify, including Rf values.(3)

(Total for Question 5 is 3 marks)

Mark scheme · C8bD Chromatography and Instrumental Analysis: Exam Drill

Question 1

(a) B1 ratio that compares how far a substance has moved relative to the solvent front in chromatography, oe
(a) Answer: Rf is the ratio comparing how far a substance moves relative to the solvent front on the chromatography paper.
(b) B1 Rf = distance moved by substance / distance moved by solvent front cao
(b) Answer: Rf = distance moved by substance / distance moved by solvent front

Question 2

(a) M1 substitutes: Rf = 42 / 84
(a) A1 0.50 cao
(a) Answer: 0.50

Question 3

(a) B1 use smaller or more concentrated spots so less sample is applied, oe
(a) B1 allow the spots to dry between applications or use a weaker solvent to reduce over-wetting, oe
(a) Answer: Any two of: apply smaller or more concentrated spots; allow spots to dry between applications; use a less aggressive solvent; place origin above solvent level.

Question 4

(a) B1 states an advantage, e.g. better resolution/separation, faster run time, or can be used with smaller sample sizes
(a) B1 explains the reason, e.g. finer particle coating on plate gives more uniform capillary action and sharper spots, oe
(a) B1 links to outcome, e.g. so similar compounds can be separated or results are obtained more quickly, oe
(a) Answer: TLC gives better resolution because the uniform thin coating causes sharper, more uniform movement of compounds, so similar compounds are separated more clearly.

Question 5

(a) M1 calculates Rf for unknown: (40 - 10) / (70 - 10) = 30 / 60 shown or equivalent
(a) M1 calculates Rf for known: (42 - 10) / (70 - 10) = 32 / 60 shown or equivalent
(a) A1 concludes that the dye is unlikely to be the same because Rf values differ (0.50 vs 0.53) and are not identical, oe
(a) Answer: Rf unknown = (40 - 10) / (70 - 10) = 30 / 60 = 0.50; Rf known = (42 - 10) / (70 - 10) = 32 / 60 = 0.53. The values differ (0.50 vs 0.53) so the known dye is unlikely to be present; the Rf values are not identical.

Chromatography and Instrumental Analysis: Exam Drill - Worksheets, Questions and Revision