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CAIE A-Level Chemistry AS 22.2.3 The Molecular Mass of an Organic Molecule Questions

Practise CAIE 9701 using molecular-ion and M+2 peaks to determine relative molecular mass and deduce an organic molecular formula.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

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

CAIE A-Level Chemistry AS 22.2.3 The Molecular Mass of an Organic Molecule Questions question 1

[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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