H3 Chemistry 9813 · Study focus: H3 Chemistry: Identify a molecular-ion peak

H3 Chemistry: Identify a molecular-ion peak

Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.

Your success criteria

Identify a molecular-ion peak
Use named chemical evidence.
Transfer the governing reason to an unfamiliar case.

Diagnose this objective

Find the intact formula mass

A molecular ion is the intact molecule after one-electron removal, M+•.

For C4H10O its nominal m/z is74 when singly charged.

Explore this H3 topic and lesson sequence.

Distinguish parent from fragments

The molecular ion retains every atom; m/z59 and31 in a C4H10O spectrum are fragments.

The highest abundant peak need not be M+•.

M+1 is an isotopic partner of M, not a second molecular formula.
For a chlorine compound, M+2 can be an isotope partner rather than the parent.

H3 Chemistry: Identify a molecular-ion peak: move from the evidence or givens, through the governing Chemistry idea, to a conclusion that stays inside the selected course boundary.

H3 Chemistry: Identify a molecular-ion peak evidence representation. Fixed sticks separate intact composition, abundance and isotope satellites.

Text alternative: Text gives formula, m/z, intensity and role.

Use isotope satellites correctly

M+1 is an isotopic partner of M, not a second molecular formula.

For a chlorine compound, M+2 can be an isotope partner rather than the parent.

Assign C4H10O

Peaks31,59,74 include M+• at74 because its nominal mass matches the formula.

Peak31 may be CH2OH+ but cannot be the intact parent.

Choose the parent.

Open the feedback checkpoint after attempting

Credit full formula mass and intact-ion definition.

Reject a contaminant

A small peak at m/z91 cannot be M+• for supplied C5H10O, Mr86.

Check calibration/contamination instead of choosing the largest m/z blindly.

Evaluate m/z91.

Open the feedback checkpoint after attempting

Credit formula mismatch.

Start the diagnostic and follow its feedback

Recognise weak parent

A weak m/z86 peak in a C5H10O spectrum can still be M+• while m/z43 is base peak.

Intensity reports abundance, not intactness.

Assign both roles.

Open the feedback checkpoint after attempting

Credit86 parent and43 fragment.

Repair highest-peak rule

An isotope satellite or contaminant can lie above M; rapid fragmentation can make M weak.

Formula, cluster spacing and plausible fragments decide the parent.

Repair: ‘highest m/z is always M.’

Open the feedback checkpoint after attempting

Require at least two counterexamples.

Cross-check the whole spectrum

For C5H10O, M+•86 and losses to71 (−15) and43 support a coherent assignment.

Next: isotope patterns at /learning/h3-mass-spectrometry-isotope-patterns-lesson.html.

Audit the assignment.

Open the feedback checkpoint after attempting

Credit formula mass and plausible lower fragments.