Define relative isotopic mass.

The mass of an atom of an isotope relative to 1/12 of the mass of an atom of carbon-12.

Define relative atomic mass.

The weighted mean mass of an atom of an element relative to 1/12 of the mass of an atom of carbon-12.

Why is relative atomic mass usually not a whole number?

It is a mean of the isotope masses weighted by their abundances.

What does relative formula mass apply to?

Compounds with giant structures (ionic or giant covalent), using the formula unit.

Write the formula for calculating Ar from mass spectrometry data.

Ar = Σ(isotopic mass × abundance) ÷ Σ(abundance).

What does m/z stand for?

Mass-to-charge ratio of an ion.

What is the molecular ion?

The ion formed when a molecule loses one electron, M⁺.

How does the molecular ion give Mr?

Its m/z equals the relative molecular mass, as the charge is 1+ and the electron mass is negligible.

What are the molecular ion peaks for Cl₂ and their ratio?

m/z 70, 72 and 74 in the ratio 9 : 6 : 1.

What are the molecular ion peaks for Br₂ and their ratio?

m/z 158, 160 and 162 in the ratio 1 : 2 : 1.

Why does a diatomic molecule give several molecular ion peaks?

Different combinations of isotopes in the molecule have different masses.

Relative masses and mass spectrometryEdexcel A-Level Chemistry: Flashcards

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What standard is the relative atomic mass scale based on?

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What standard is the relative atomic mass scale based on?: 1/12 of the mass of an atom of carbon-12.
Define relative isotopic mass.: The mass of an atom of an isotope relative to 1/12 of the mass of an atom of carbon-12.
Define relative atomic mass.: The weighted mean mass of an atom of an element relative to 1/12 of the mass of an atom of carbon-12.
Why is relative atomic mass usually not a whole number?: It is a mean of the isotope masses weighted by their abundances.
What does relative formula mass apply to?: Compounds with giant structures (ionic or giant covalent), using the formula unit.
Write the formula for calculating Ar from mass spectrometry data.: Ar = Σ(isotopic mass × abundance) ÷ Σ(abundance).
What does m/z stand for?: Mass-to-charge ratio of an ion.
What is the molecular ion?: The ion formed when a molecule loses one electron, M⁺.
How does the molecular ion give Mr?: Its m/z equals the relative molecular mass, as the charge is 1+ and the electron mass is negligible.
What are the molecular ion peaks for Cl₂ and their ratio?: m/z 70, 72 and 74 in the ratio 9 : 6 : 1.
What are the molecular ion peaks for Br₂ and their ratio?: m/z 158, 160 and 162 in the ratio 1 : 2 : 1.
Why does a diatomic molecule give several molecular ion peaks?: Different combinations of isotopes in the molecule have different masses.

Exam questions on Relative masses and mass spectrometry

A sample of magnesium was analysed by mass spectrometry. It contains three isotopes: ²⁴Mg (78.6%), ²⁵Mg (10.1%) and ²⁶Mg (11.3%).
Explain why the relative atomic mass of magnesium is not a whole number, even though the mass number of each isotope is a whole number.2 marks
Chlorine consists of two isotopes, ³⁵Cl and ³⁷Cl, in an abundance ratio of 3 : 1. A sample of chlorine gas, Cl₂, is analysed in a mass spectrometer in which each molecule is ionised to form a Cl₂⁺ ion.
Predict the relative heights of the peaks in the Cl₂⁺ region of the mass spectrum of chlorine. Show your working.2 marks
A sample of copper from a coin was analysed by mass spectrometry. The spectrum of Cu⁺ ions has two peaks: m/z 63 with relative abundance 69.1 and m/z 65 with relative abundance 30.9. A second sample of copper, from a meteorite, has a relative atomic mass of 63.75.
Calculate the relative atomic mass of the copper from the coin. Give your answer to 3 significant figures.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).