Do OCR A students need time-of-flight mass spectrometry?

No. The isotope and mass-spectrum practice here keeps TOF mechanics out of OCR A. The separate TOF calculation trainer is labelled AQA only.

Are hard isotope algebra questions useful for OCR A?

Yes. They are especially characteristic of AQA, but unfamiliar isotope algebra is useful stretch practice for OCR A and can support tougher synoptic questions.

Why do some weighted-mean questions not divide by 100?

If the abundances are percentages of the whole element they total 100. If the data are relative values or describe only one component of a mixture, divide by the total abundance represented by the data you are averaging.

AQA · OCR A · Year 12

Isotope questions and mass spectrometry practice

Choose a topic or try a mixed set. Practise relative atomic mass, isotope abundance, mass spectra and the harder algebra that sometimes appears in exams.

Relative atomic mass questions Mixed questions

⁶³X⁶⁵X⁶⁶X

weighted meanmass × abundance, divided by total abundance

Exam board

TOF mass spectrometry is AQA only. OCR isotope practice here never includes TOF mechanics.

Choose a topic

Isotope basicsDefinitions, sub-atomic particles, mass number and chemical properties. Practise Relative atomic massWeighted means from isotope abundances, accurate isotope masses and mass-spectrum data. Practise Find an abundanceWork backwards from Aᵣ to find missing isotope abundances. Practise Find the isotopeFind a missing isotope mass or mass number from Aᵣ and abundance data. Practise Mass-spectrum patternsRead isotope peaks, relative abundance and molecular-ion isotope patterns. Practise Hard isotope algebraMulti-step isotope algebra with ratios, unknowns and awkward normalisation. Practise Mixed isotope practiceA mixture of questions from across the isotope and mass-spectrometry topic. Practise

Relative atomic mass

A_r = Σ(mass × abundance) ÷ Σ(abundance)

If the abundances are percentages of the whole sample, divide by 100. If they only describe part of a mixture, divide by their actual total instead.

Reading a mass spectrum

For singly charged isotope ions, m/z gives the isotope mass and peak size gives relative abundance.

Molecular ions containing several atoms can give characteristic isotope patterns.

Want a harder one?

The hard set adds multi-step algebra, ratios, missing values and awkward normalisation. It is especially useful for AQA and worthwhile OCR stretch practice.

Hard isotope questions