AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Mass spectrometry can be used to determine the molecular formula of a…
Full specification wording
Mass spectrometry: Mass spectrometry can be used to determine the molecular formula of a compound.
In a mass spectrometer a molecule loses one electron to become the molecular ion, so the peak at the highest mass to charge ratio, ignoring small isotope peaks beyond it, gives the relative…
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 2
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a mass spectrometer a molecule loses one electron to become the molecular ion, so the peak at the highest mass to charge ratio, ignoring small isotope peaks beyond it, gives the relative molecular mass directly. That alone leaves several possible formulae, because many combinations share the same whole-number mass. A high resolution instrument measures the mass to four or five decimal places, and because the precise masses of individual isotopes are not whole numbers, formulae that appear identical at whole-number resolution separate cleanly. What the technique cannot do is separate isomers: two isomers have exactly the same molecular formula and therefore exactly the same precise molecular mass, which is why AQA offers infrared fingerprinting instead in that situation.
What earns the marks
- Identifying the molecular ion peak as the peak at the highest mass to charge ratio.
- Equating the mass to charge ratio of that peak with the relative molecular mass.
- Saying precise, rather than whole-number, atomic masses must be used.
- Selecting the formula whose calculated precise mass matches the measured value.
Common mistakes and how to put them right
Reading the base peak, the tallest one, as the molecular ion peak.
Rounding the precise masses, which destroys the discrimination the method depends on.
Expecting high resolution mass spectrometry to distinguish two isomers.
Saying the molecular ion is positive because a proton has been added rather than an electron removed.
Exam tips
- State that the molecular ion has lost one electron, so its mass is effectively that of the molecule.
- Work to as many decimal places as the data gives you, and only round at the very end.
- If a question asks you to distinguish isomers, reach for the infrared fingerprint region, not the mass spectrum.
Try the lesson
The start of the Mass spectrometry: Mass spectrometry can be used to determine the molecular formula of a compound. lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion in a mass spectrometer gives the relative molecular mass of a compound?
Listen
In a mass spectrometer a molecule loses one electron to become the molecular ion, so the peak at the highest mass to charge ratio, ignoring small isotope peaks beyond it, gives the relative molecular mass directly. That alone leaves several possible formulae, because many combinations share the same whole-number mass. A high resolution instrument measures the mass to four or five decimal places, and because the precise masses of individual isotopes are not whole numbers, formulae that appear identical at whole-number resolution separate cleanly. What the technique cannot do is separate isomers: two isomers have exactly the same molecular formula and therefore exactly the same precise molecular mass, which is why AQA offers infrared fingerprinting instead in that situation. A strong answer earns marks for these points. identifying the molecular ion peak as the peak at the highest mass to charge ratio. equating the mass to charge ratio of that peak with the relative molecular mass. saying precise, rather than whole-number, atomic masses must be used. selecting the formula whose calculated precise mass matches the measured value. Exam tip: State that the molecular ion has lost one electron, so its mass is effectively that of the molecule.
Key words
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molecular ion peak
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high resolution
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isomers not distinguished
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See every definition
- molecular ion peak
- The peak at the highest mass to charge ratio, ignoring small isotope peaks beyond it, which gives the relative molecular mass directly.
- high resolution
- A property of an instrument that measures the mass to four or five decimal places, so formulae that appear identical at whole-number resolution separate cleanly.
- isomers not distinguished
- Two isomers have exactly the same molecular formula and therefore exactly the same precise molecular mass, so the technique cannot separate them.
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