Irvington High School  AP Chemistry
Mr. Markic
Name______________________________
Number __ Date __/__/__
3  Mass Relationships in Chemical Reactions
M a s s
S p e c t r o m e t r y
1. Carbon has two stable isotopes, 12C6 and 13C6, and fluorine has only one stable isotope, 19F9. How
many peaks would you observe in the mass spectrum of the positive ion of CF4+. Assume that the
ion does not break up into smaller fragments.
Since there are only two isotopes of carbon, there are only two possibilities for CF4.
12 19 
6 C 9 F4
(molecular mass 88 amu) and 136 C 199 F4 (molecular mass 89 amu)
There would be two peaks in the mass spectrum.
2. Hydrogen has two stable isotopes, 1H1, and 2H1, and sulfur has four stable isotopes, 32S16, 33S16,
34
S16, 36S16. How many peaks would you observe in the mass spectrum of the positive ion of
hydrogen sulfide, H2S+? Assume no decomposition of the ion into smaller fragments.
Since there are two hydrogen isotopes, they can be paired in three ways: 1H-1H, 1H-2H, and 2H2
H. There will then be three choices for each sulfur isotope. We can make a table showing all the
possibilities (masses in amu):
32
33
34
36
S
S
S
S
1
H2
34
35
36
38
1 2
HH
35
36
37
39
2
H2
36
37
38
40
There will be seven peaks of the following mass numbers: 34, 35, 36, 37, 38, 39, and 40.
Very accurate (and expensive!) mass spectrometers can detect the mass difference between two 1H
and one 2H. How many peaks would be detected in such a “high resolution” mass spectrum?
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