Edexcel A-Level Chemistry AS 2.6 The Basic Principles of a Mass Spectrometer Questions

Order ionisation, acceleration, deflection and detection, then multiply each isotope mass by its abundance and use peak positions and heights in the conclusion.

Exam points

Order ionisation, acceleration, deflection and detection in a mass spectrometer.
Calculate relative atomic mass from isotope masses and percentage abundances.
Interpret mass-to-charge peaks and heights to deduce isotopic composition.

[Maximum number: 5]

This question is about magnesium, magnesium oxide and magnesium sulfate.

(a)

A sample of magnesium contains three isotopes and has a relative atomic mass of 24.32.
The table gives the relative abundances of two of these isotopes.

Table for Question (a) — Edexcel A-Level Chemistry AS

[ 5 ]

(i)

Calculate the relative abundance and hence the mass number of the third isotope.
Give your answer to the appropriate number of significant figures.
You must show all your working.

[ 4 ]

An answer that makes reference to the following points:

- calculation of the \% abundance of the third isotope

(1)

- substitute equation

(1)

- calculation of the mass of the $3^{\text {rd }}$ isotope

(1)

- answer to 2 SF only

(1)

Example of calculation

100-78.99-10.00=11.01(%)

100-78.99-10.00 = 11.01(\%)

$24.32=\frac{(24 \times 78.99)+(25 \times 10)+(y \times 11.01)}{100}$ $\mathrm{y}=\frac{2432-(24 \times 78.99)-(25 \times 10)}{11.01}$

y=25.998

(4)

(ii)

State which of these isotopes would be deflected most in a mass spectrometer. Justify your answer.

[ 1 ]

An answer that makes reference to the following point:

- ${ }^{24} \mathrm{Mg}$ and lowest mass or lowest m / z ratio (so deflected more by the magnetic field)

Allow ${ }^{24} \mathrm{Mg}$ is lightest Allow ${ }^{24} \mathrm{Mg}^{+}$

Ignore just the lowest mass

(1)

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Edexcel A-Level Chemistry AS 2.6 The Basic Principles of a Mass Spectrometer Questions question 1

Question (a)

Question (i)

Question (ii)