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CAIE A-Level Chemistry AS 22.2 Mass Spectrometry Questions

Practise interpreting isotope, molecular-ion, fragment, M+1 and M+2 peaks to obtain composition and structure.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • use m/e positions and peak abundances to identify isotopes and calculate relative atomic mass
  • read the molecular ion for Mr and use fragment-ion formulae to distinguish candidate structures
  • apply M+1 for carbon count and M+2 abundance patterns for chlorine or bromine

Covered syllabus points

22.2.1—Analyse mass spectra22.2.2—The atomic mass of an element22.2.3—The molecular mass of an organic molecule22.2.4—Suggest the identity of molecules formed22.2.5—The number of carbon atoms, n, in a compound22.2.6—The presence of bromine and chlorine atoms

Question 1

[Maximum number: 1]

Oxygen has three stable isotopes, 16O,17O{ }^{16} \mathrm{O},{ }^{17} \mathrm{O} and 18O{ }^{18} \mathrm{O}. All three isotopes are present in a sample of oxygen gas, O2\mathrm{O}_{2}, which was analysed using a mass spectrometer.

How many peaks associated with the O2+\mathrm{O}_{2}{ }^{+}ion would be expected?

A

3

B

5

C

6

D

9

Question 2

[Maximum number: 1]

The relative atomic mass of antimony is 121.76 .
Antimony has two isotopes. The mass numbers of the two isotopes differ by two. The isotope with the lower mass number is the more abundant.

What is the percentage abundance of the isotope with the higher mass number?

A

12 %

B

38\%

C

62 %

D

88\%

Question 3

[Maximum number: 1]

Organic compounds can be distinguished using chemical tests and analytical techniques.

Organic compound E contains three carbon atoms. E reacts with cold dilute acidified KMnO4(aq)\mathrm{KMnO}_{4}(\mathrm{aq}) to form a single compound F with Mr=154.9M_{\mathrm{r}}=154.9.
Fig. 3.1 shows the infrared spectrum of E.
Fig. 3.2 shows the infrared spectrum of F.

Fig. 3.1

Fig. 3.1

Fig. 3.2

Fig. 3.2

Table 3.2

Table 3.2

Both spectra show absorptions between 2850 and 2950 cm−12950 \mathrm{~cm}^{-1} owing to C-H bonds in each molecule.

The mass spectrum of E shows a molecular ion peak and an M+2 peak of approximately equal abundance at m / e=120 and 122 .

Deduce the relative molecular mass, MrM_{\mathrm{r}}, of E.

Mr=M_{r}=
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