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.)

I figured the first one out... it's 108 and 110

but the second part I keep getting wrong. I got 49,51 for the percentages.

Explanation / Answer

Molar mass of C2H5Br (with 79 Br) = 2 * 12 + 5 * 1 + 79 = 108 gm/mol

Molar mass of C2H5Br (with 81 Br) = 2 * 12 + 5 * 1 + 81 = 110 gm/mol

Hence, for the first part correct answer is 108,110

The percentage of each molecular ion will be in the same ratio as of Br, since the Br is the only compound with different masses

Hence the correct answer is 50.7% and 49.3%

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