1. * a student wants to find out which pigments are in a

1. * A student wants to find out which pigments are in a plant.

She does a chromatography experiment on a sample from the plant.

Look at her results.

The Rf values for some pigments are shown in the table.

Pigment Rf value

A 0.95

B 0.45

C 0.32

D 0.25

E 0.15

Calculate the Rf value for each spot.

Describe and explain which pigments are in the sample from the plant and suggest why further analysis of theplant pigments is needed.

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Mark Scheme

1 Level 3 (5–6 marks)Demonstrates knowledge of the formula forRf and applies knowledge andunderstanding to calculate all Rf valuescorrectly.ANDCorrectly analyses the results obtained andassigns spots to pigments.ANDAnalyses the results to suggest why furtheranalysis of the plant pigments is needed

There is a well-developed line of reasoningwhich is clear and logically structured. Theinformation presented is relevant andsubstantiated.

Level 2 (3–4 marks)Demonstrates knowledge of the formula forRf and applies knowledge andunderstanding to calculate most of the Rfvalues correctly.ANDCorrectly analyses the results obtained andassigns at least 2 spots to pigments.

There is a line of reasoning presented withsome structure. The information presentedis relevant and supported by someevidence.

Level 1 (1–2 marks)Demonstrates knowledge of the formula forRf and applies knowledge andunderstanding to calculate some of the Rfvalues correctly.ORAnalyses their results to suggest whyfurther analysis of the plant pigments isneeded

There is an attempt at a logical structurewith a line of reasoning. The information isin the most part relevant.

0 marksNo response or no response worthy ofcredit.

6(AO1 ×

1.2)(AO1 ×

2.2)(AO2 ×3.2b)

(AO2 ×3.3b)

AO1.2 Demonstrates knowledge of theformula to calculate Rf values.

AO2.2 Applies knowledge andunderstanding of formula to calculate Rfvalues for the 4 spots

Spot Rf value Allow1 0.13 0.1252 0.32 0.318 /

0.31823 0.65 0.648 /

0.64774 0.95 0.955 /

0.9545

At L1 & L2 IGNORE rounding errors

AO3.2b Analyses information to drawconclusions about the pigments:Spot 1 = Could be Pigment E, because it isclosest but cannot confirm, as Rf valuedoes not match exactly.Spot 2 = Pigment CSpot 3 = unknownSpot 4 = Pigment A

AO3.3b Analyses information to identifyimprovements that could be made in orderto identify spot 3.Look up Rf values of other pigments inorder to match to spot 3Further investigation needed if Rf value notfound

Examiner’s CommentsSome candidates knew how to calculate Rfvalues and made a reasonable attempt tostate the Rf formula and to calculate someor all of the Rf values correctly. Roundingerrors stopped some candidates accessinghigher levels. Not all identified pigments Aand C, although some knew they werepresent but did not link them to spots in thechromatogram. A few higher abilitycandidates accessed Level 3 by explaining


Question Answer/Indicative content Marks Guidance

Mark Scheme

that spot 3 did not relate to any of theknown pigments that had been tested, butmost did not realise that the identity of spot1 is ambiguous as its Rf value did notmatch any pigments exactly. Most simplyrestated the stem of the question to saythat further analysis is needed, but did notclearly suggest why or what should bedone. Virtually no one suggested lookingup the Rf values of other pigments.

Many candidates did not attempt thisquestion, and some just wrote irrelevantfacts about plant pigments used forphotosynthesis. Some candidates gainedlittle credit as they were unable to presenttheir answers in a coherent and logicalway.

Exemplar 1

This candidate has measured the solventfront accurately and used the Rf formula tocalculate all four Rf values correctly. Theyhave analysed the information to concludethat pigments A and C were in the sample.However, they have not explained whichspots they relate to on the chromatogram.They state that further analysis is needed(as given in the stem of the question), buthave not made any relevant suggestionsas to why this is necessary. This responsewas credited Level 2 with 3 marks. Withimproved communication to link spot 4 topigment A, and spot 2 to pigment C thecandidate would have been credited Level2 with 4 marks.

Total 6

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