CAIE A-Level Chemistry 22.2 Mass Spectrometry

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

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 spectra 22.2.2—The atomic mass of an element 22.2.3—The molecular mass of an organic molecule 22.2.4—Suggest the identity of molecules formed 22.2.5—The number of carbon atoms, n, in a compound 22.2.6—The presence of bromine and chlorine atoms

[Maximum number: 2]

Copper is used in electrical equipment. It has a melting point of $1085^{\circ} \mathrm{C}$ .

(a)

The relative isotopic masses and natural abundances of the two isotopes in a sample of copper are shown in Table 1.1.

Table 1.1

[ 1 ]

(i)

Calculate the relative atomic mass, $A_{\mathrm{r}}$ , of copper in this sample using the data in Table 1.1.

Show your working.

A_{r}=

[ 1 ]

$(62.93 \times 69.15) \div 100+(64.928 \times 30.85) \div 100=63.55 / 63.546$

(b)

The mass spectrum of a sample of pure copper is shown in Fig. 1.1.

Fig. 1.1

Identify the ion with an abundance of 23 % in the sample.

[ 1 ]

${ }^{63} \mathrm{Cu}^{2+}$

[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 $\mathrm{KMnO}_{4}(\mathrm{aq})$ to form a single compound F with $M_{\mathrm{r}}=154.9$ .
Fig. 3.1 shows the infrared spectrum of E.
Fig. 3.2 shows the infrared spectrum of F.

Fig. 3.1

Fig. 3.2

Table 3.2

Both spectra show absorptions between 2850 and $2950 \mathrm{~cm}^{-1}$ 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, $M_{\mathrm{r}}$ , of E.

M_{r}=

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Covered syllabus points

22.2 Mass spectrometry question 1

Question (a)

Question (i)

Question (b)

22.2 Mass spectrometry question 2