---
title: "Mass Spectra and Infrared: Identifying an Unknown"
description: "A mass spectrum gives the molecular mass from the molecular ion peak and hints at the structure from the fragments. An infrared spectrum shows which bonds are present by the frequencies they absorb."
canonical: https://lightmysky.com/learn/science/mass-spectra-and-infrared-identifying-an-unknown-mt_yotOEg9OTn
source: https://lightmysky.com/learn/science/mass-spectra-and-infrared-identifying-an-unknown-mt_yotOEg9OTn.md
retrieved: 2026-09-12
---

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# Mass Spectra and Infrared: Identifying an Unknown

A mass spectrum gives the molecular mass from the molecular ion peak and hints at the structure from the fragments. An infrared spectrum shows which bonds are present by the frequencies they absorb.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/mass-spectra-and-infrared-identifying-an-unknown-mt_yotOEg9OTn

## Ready when they can

- Reads the relative molecular mass off the molecular ion peak and accounts for the M plus 1 and M plus 2 peaks
- Suggests the fragment lost from the gap between two peaks
- Matches absorptions to hydroxyl, carbonyl and carbon-oxygen bonds using a data table
- Combines both spectra to choose between two candidate structures

## Lesson: Reading masses and bond fingerprints

A mass spectrum sorts ions by mass and records how much of each arrives. You read the relative molecular mass off the highest mass peak, ignoring tiny isotope bumps, so you always look at the far right end first.

**Example.** You also read the gaps between big peaks, since each gap equals a lost fragment. A gap of 15 means a methyl group broke off, and a gap of 29 means an ethyl group. Small bumps one and two units higher come from heavier isotopes, and their sizes hint at which atoms sit inside.

An infrared spectrum maps which bonds a molecule holds, since each bond soaks up its own frequency. You check three regions with a data table: a broad absorption near 3300 means an O H hydroxyl group, a sharp strong one near 1700 means a C=O carbonyl group, and bands near 1100 point to C O single bonds.

**Tip.** You choose between candidate structures by matching every headline peak. If a candidate needs a bond the spectrum lacks, or the spectrum shows one it cannot explain, you reject it. A molecular ion at 60 with a broad band near 3000 and a sharp one near 1700 points to a small acid carrying both O H and C=O.

**Recap.** The far right mass peak gives the mass, gaps name lost fragments, and IR bands name the bonds.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Empirical and Molecular Formulas from Composition](https://lightmysky.com/learn/science/empirical-and-molecular-formulas-from-composition-mt_4rN5MQaTf7)
- [Isomerism: Same Formula, Different Molecule](https://lightmysky.com/learn/science/isomerism-same-formula-different-molecule-mt_DvD0coeKZe)
- [Addition and Condensation Polymers](https://lightmysky.com/learn/science/addition-and-condensation-polymers-mt_klu208vTI1)
- [Isotopes and Relative Atomic Mass](https://lightmysky.com/learn/science/isotopes-and-relative-atomic-mass-mt_NFoRZd6UN_)

## Opens up

- [Reading Bond Lengths and Force Constants from Spectra](https://lightmysky.com/learn/science/reading-bond-lengths-and-force-constants-from-spectra-mt_5m7J1mFw68)
- [NMR Spectra: Counting Environments](https://lightmysky.com/learn/science/nmr-spectra-counting-environments-mt_JquoiaxOMu)
- [Mass Spectrometry: Ionisation Methods and Mass Analysers](https://lightmysky.com/learn/science/mass-spectrometry-ionisation-methods-and-mass-analysers-mt_JTTb7FM1L4)