HKDSE Chemistry: Instrumental Analysis IR, Mass Spectrometry and NMR Tutor Prep Guide

HKDSE Chemistry: Instrumental Analysis IR, Mass Spectrometry and NMR Tutor Prep Guide | Three-Step Reading

2026-11-10

Tutor lesson-prep guides · Secondary Chemistry

Put three spectra in front of a student and most read left to right, then remember only how many peaks there were. The problem is order, not knowledge: IR finds functional groups, mass spectrometry fixes the molecular mass and fragments, NMR counts hydrogen environments. Fix the order and the spectra stop being guesses.

Objectives — Students can: (1) find a carbonyl and a hydroxyl on an IR spectrum in a fixed order and quote the characteristic absorption regions; (2) read the relative molecular mass from the molecular ion peak, estimate the carbon count from the M+1 peak, and recognise chlorine and bromine from 3 to 1 and 1 to 1 doublets; (3) use count peaks, read integrals, read chemical shift to deduce structural fragments from a proton NMR spectrum.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Functional groups	Which two groups does a carboxylic acid contain?	Carbonyl and hydroxyl
Relative molecular mass	What is the relative molecular mass of propane?	44
Isotopes	What is the approximate abundance ratio of the two chlorine isotopes?	About 3 to 1

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Three spectra	One unknown with IR, mass spectrum and NMR	Say which you would read first and why
6-20 min	IR in three steps	Carbonyl, then hydroxyl, then carbon-hydrogen region	Circle the two decisive absorptions
20-34 min	Mass spectrometry	Molecular ion, fragments, M+1 and halogen doublets	Work out the molecular mass
34-48 min	NMR in three steps	Count peaks, read integrals, read shift and splitting	Write the hydrogen ratio of three environments
48-56 min	Three into one	Build an evidence chain from the three spectra	Deduce one simple structure independently
56-60 min	Exit check	Collect two questions and mark at once	Complete independently

03 Teaching script (read it out)

Scene 1: IR: find two things, leave the rest to the fingerprint region

Students sweep the whole IR left to right and treat every trough as a functional group. Fix the order: first the carbonyl, a strong sharp absorption near 1700 wavenumbers; then the hydroxyl, a broad absorption from about 2500 to 3500; only then the carbon-hydrogen region to separate saturated from unsaturated. Broad against sharp is the decisive contrast: a carboxylic acid hydroxyl is broad and spans hundreds of wavenumbers, an alcohol hydroxyl is broad but a little higher, and both stand against the sharp carbonyl. Below about 1500 wavenumbers the fingerprint region is used only to decide whether two samples are identical, never to name every peak. Give one counterexample: a spectrum with a sharp carbonyl but no broad hydroxyl is a ketone or an aldehyde, not a carboxylic acid. That single line saves a lot of marks.

Students do: circle carbonyl and hydroxyl on three spectra and state the difference in one sentence.

Scene 2: Mass spectrometry: the molecular ion gives mass, fragments give the skeleton

Step one: read the highest mass-to-charge peak on the right, usually the molecular ion, whose value is the relative molecular mass. Worked example: a molecular ion at 58 with a fragment at 43 differs by 15, exactly one methyl group, so the skeleton starts to appear. Step two: read the isotopes. The M+1 peak comes mainly from carbon-13 at about 1.1 percent per carbon, so three carbons give about 3.3 percent, and the height of M+1 lets you count carbons. This is the calculation the exam repeats most. Step three: read halogen fingerprints. Chlorine gives a doublet of roughly 3 to 1, bromine roughly 1 to 1, both two mass units apart, so a paired peak means a halogen is present. Finally warn that the molecular ion need not be the tallest peak; the base peak is often a fragment, so never take the highest peak as the molecular mass.

Students do: take one step from the molecular ion and one from a fragment, naming the group lost each time.

Scene 3: NMR: peak count is environments, integral is the hydrogen ratio

The order is fixed: count the sets of peaks to get the number of hydrogen environments; read the areas to get the ratio of hydrogens; then use chemical shift to decide what each set is attached to. Only a few anchors are needed: tetramethylsilane at 0, alkyl hydrogens near 1, hydrogens on a carbon bonded to oxygen about 3.5 to 4, an aldehyde hydrogen about 9 to 10, and a carboxylic acid hydrogen about 10 to 12 as a single broad peak. Splitting follows n plus 1: n hydrogens on the adjacent carbon split the peak into n plus 1 lines, so a triplet means two neighbours. Apply the three steps to ethanol: three sets of peaks, integrals 3 to 2 to 1, and shifts rising from methyl through methylene to hydroxyl. Students then build the evidence chain themselves instead of reciting an answer.

Students do: for ethanol and ethyl ethanoate, write the environment and hydrogen count for each set of peaks.

04 Question types and worked steps

Type	Example	Steps	Answer
Functional group identification	Decide from IR whether a carboxyl group is present	Find carbonyl, then broad hydroxyl	Both must be present before calling it an acid
Mass and fragment question	Read molecular mass and the group lost	Take the highest mass-to-charge peak, then subtract	The difference must match a real group mass
Calculation	Estimate carbon count from the M+1 height	Divide by 1.1 and round	State the assumption and the unit
Structure deduction	Combine three spectra into one structure	IR for groups, mass for mass, NMR for hydrogen environments	The three conclusions must agree

Sanity check (teach this): Sanity check: the chapter fixes a division of labour – IR for functional groups, mass spectrometry for molecular mass and fragments, NMR for hydrogen environments – so students do not shuffle three spectra into a guess; every calculation states its assumption that carbon-13 abundance is about 1.1 percent; halogen doublets are remembered as 3 to 1 and 1 to 1, which follow from isotope abundance; and the final step requires the three conclusions to agree, so a mismatch sends the student back to the most reliable step rather than to a rewritten answer.

05 Independent practice (3 questions)

Question	Answer	What to watch
IR groups	Distinguish a carboxylic acid from a ketone using two spectra	Check that the broad hydroxyl is mentioned
Mass reading	Read the molecular ion and the group lost	Check that the difference matches a methyl group
NMR fragments	From three peak sets and integrals, name the environments	Check that integrals are used

06 Common mistakes and what to say

Mistake	Student writes	Say this
Tallest peak as molecular mass	Treating the base peak as the molecular ion	Tallest is not the same as furthest right. The molecular ion is the highest mass-to-charge value, so read position, not height.
Naming every IR trough	Treating each fingerprint trough as a functional group	The fingerprint region compares two samples; it does not name groups. Find the carbonyl and the hydroxyl first.
Only counting NMR peaks	Stopping after saying there are three sets of peaks	Three sets tells you how many environments. Where is the hydrogen count? Did you read the integrals and the splitting?
Missing isotope doublets	Ignoring a 3 to 1 doublet	Two peaks two mass units apart, appearing as a pair: chlorine or bromine? That is a free mark.

07 Homework (paper, section and question numbers)

Chemistry chapter index — All chapters for this subject
Full prep guide index — Matching chapters in other subjects
Redo this chapter — Practice section, questions 2, 4 and 6: IR reading, mass calculation, NMR fragments

08 Exit check (last 5 minutes)

Question	Answer	Decision
Three jobs	IR groups, mass molecular mass, NMR hydrogen environments	Mixed up means returning to scene one
Mass in two steps	Read highest mass-to-charge, estimate carbons from M+1	Base peak as molecular mass means repeating scene two
NMR in three steps	Count peaks, read integrals, read shift	Peak count only means returning to scene three

09 Tutor FAQ

Is carbon-13 NMR needed?

The syllabus centres on proton NMR; carbon-13 only needs the idea that it counts different carbon environments.

How far must splitting be learned?

n plus 1 is enough; singlet, doublet, triplet and quartet cover the examined cases.

How many IR values must be memorised?

Three regions: carbonyl near 1700, hydroxyl 2500 to 3500, carbon-hydrogen 2850 to 3300; everything else is read by comparison.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.