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

Order ionisation, acceleration, deflection and detection, calculate relative atomic mass from isotope abundances, and interpret mass-to-charge peaks and heights.

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)