CAIE A-Level Chemistry AS 22.2.5 The Number of Carbon Atoms N in a Compound Questions

Practise CAIE 9701 M+1 isotope calculations using the carbon-13 contribution to determine carbon count or predict a peak intensity.

Exam points

substitute both relative peak abundances into n = 100 × [M+1] / (1.1 × M)
keep M and M+1 peak intensities in the same abundance scale before calculating
rearrange the same relationship to predict the M+1 abundance when carbon count and M are given

[Maximum number: 1]

Fig. 4.1 shows how propane, $\mathrm{C}_{3} \mathrm{H}_{8}$ , can be converted to propanoic acid, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ .

Fig. 4.1

$\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ reacts with an unsaturated alcohol R to form unsaturated ester S.

The mass spectrum of S shows the following peaks.

Table 4.3

Use Table 4.3 to calculate the number of carbon atoms in S.
number of carbon atoms in S=

( $0.31 / 4.7 \times 100 / 1.1$ ) = 6 carbon atoms

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