S6 Chemistry: Instrumental Analysis (Paper 3: Challenge)

2026-11-12

Ten items combining mass spectra, infrared and chemical tests to deduce structures, with dilution and percentage calculations.

Direct answers　Answers in order: ethanoic acid, C2H4O2, 2.0e-4 mol dm-3, 2.5e-3 mol dm-3, bromoethane, an ethoxy group, they confirm each other, the two carbonyls of an anhydride, 1.0 percent, no.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S6 Chemistry: Instrumental Analysis
Level	S6

02　Questions (10 items)

FoundationA compound has its molecular ion at m/z 60, a broad infrared band at 3300 and a strong band at 1710, and it releases gas with sodium hydrogencarbonate. What is it?
FoundationElemental analysis gives 40.0 percent carbon, 6.7 percent hydrogen and 53.3 percent oxygen with relative molecular mass 60. What is the molecular formula?
FoundationA solution has absorbance 0.48, molar absorption coefficient 1200 dm3 mol-1 cm-1 and path length 2.0 cm. What is its concentration?
IntermediateA sample diluted 25 times reads 1.0e-4 mol dm-3 in the cell. What is the original concentration?
IntermediateThe M peak is at m/z 109 and M+2 is of similar height. What is the compound?
IntermediateA fragment at M minus 45 appears in the mass spectrum. Which group is most likely lost?
ChallengeWhy check both retention time and the mass spectrum when identifying an unknown by GC-MS?
ChallengeAn infrared spectrum shows two strong bands at 1800 and 1750 cm-1. Which group is most likely present?
ChallengeA 0.500 g sample is made up to 250 cm3 and diluted fivefold before atomic absorption reads 4.0 ppm. What is the percentage by mass?
Error redoMisconception drill: can infrared alone establish a molecular formula?

03　Answers and one-line notes

#	Answer	One-line note
1	Ethanoic acid	The gas from hydrogencarbonate proves a carboxylic acid group, and m/z 60 matches C2H4O2.
2	C2H4O2	60 times 0.40 over 12 gives two carbons, 60 times 0.067 gives four hydrogens and 60 times 0.533 over 16 gives two oxygens.
3	2.0e-4 mol dm-3	Divide 0.48 by the product of 1200 and 2.0.
4	2.5e-3 mol dm-3	Dilution divides the concentration, so recovering it multiplies by 25.
5	Bromoethane, C2H5Br	Bromine gives M and M+2 of similar height, and 24 plus 5 plus 80 gives 109.
6	An ethoxy group, OC2H5	45 is two carbons, five hydrogens and one oxygen, the mass of an ethoxy group.
7	The two confirm each other and avoid co-elution errors	An impurity with the same retention time can co-elute, while the mass spectrum gives a structural fingerprint, so only agreement is trustworthy.
8	The two carbonyls of an anhydride	An anhydride has two coupled carbonyls, so two strong bands appear close together.
9	1.0 percent	4.0 ppm times 5 gives 20 ppm in the made up solution, 0.25 L holds 5 mg, and 5 mg in 500 mg is 0.01.
10	No, mass spectrometry and elemental analysis are also needed	Infrared only shows functional groups; the formula needs the molecular ion mass together with the element ratios.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

In combined spectra questions, write the relative molecular mass first, then check each functional group against it.Tutor Registration →

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