Halogenated compounds are particularly easy to identify by their mass spectra be

Question

Halogenated compounds are particularly easy to identify by their mass spectra because chlorine and bromine occur naturally as mixtures of two abundant isotopes. Chlorine occurs as 35Cl (75.8%) and 37Cl (24.2%); Bromine occurs as 79Br (50.7%) and 81Br (49.3%); Boron compounds also stand out owing to the two isotopes 10B (19.9%) and 11B (80.1%). For the compound Bromoethane, C2H5Br:

At what masses do the molecular ions occur? (List in order of increasing mass separated by commas, e.g. 120,122.) What are the percentages of each molecular ion? (List to nearest 1% in order of increasing mass separated by commas, e.g. 55,45.)

Explanation / Answer

Bromoethane, C2H5Br

Let’s figure out the molecular weight for each isotope of bromine:

For 79Br: (2*12.01) + (5*1.008) + 79 = 108.06 (at 50.7%)

For 81Br: (2*12.01) + (5*1.008) + 81 = 110.06 (at 49.3%)

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