CAIE A-Level Chemistry 22.2.3 Molecular Mass from Mass Spectra

Practise identifying the molecular-ion peak and using its m/e value and isotope cluster to determine relative molecular mass.

Exam points

identify M+ as the intact molecular radical cation rather than automatically choosing the tallest peak
for a singly charged molecular ion, use its m/e value as the nominal molecular mass
use the centre of a chlorine or bromine isotope cluster when a non-integer relative molecular mass is required

[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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22.2.3—The molecular mass of an organic molecule question 1