[philonossis]

"Chlorine has two isotopes with mass number 35 and 37.  Combination of chlorine with bromine produces BrCl.  An examination of the product BrCl molecules shows the presence of only three isotopically different types of molecules with approximate masses of 114, 116, and 118 amu.  Based on this information, how many isotopes of Br exist and what are the mass numbers of the isotopes?"

My method:
Take the 3 mass number of BrCl and subtract both mass numbers of Cl from each.  This gives six results:

114 (BrCl) - 35 (Cl) = 79; 116 (BrCl) - 35 (Cl) = 81;  118 (BrCl) - 35 (Cl) = 83

114 (BrCl)  - 37(Cl) = 77;  116 (BrCl)  - 37(Cl) = 79;  118  (BrCl) - 37(Cl) = 81

So I got 4 isotopes of Br with mass numbers 77, 79, 81, 83

The book says 2 isotopes for Br: 79 and 81

I would like to know how to figure this if anyone could offer a suggestion.  

Is the rule of thumb to multiply the isotopes of one element times the isotopes of the other element?

In my method, I came up with 2 matching sets, 79 and 81.

[Jorriss]

118 - 35 is 83 amu, except if you combined 83 from Bromine with 37 from Chlorine you would get a compound with a mass of 120 amu's, which there isn't! Which means 83 can't be one.

[DrCMS]

118 - 35 is 83 amu, except if you combined 83 from Bromine with 37 from Chlorine you would get a compound with a mass of 120 amu's, which there isn't! Which means 83 can't be one.

Exactly the same at the other end 77 + 35 would be 112 which is not part of the pattern so 77 is not an isotope of Bromine.

118 comes from 81Br + 37Cl
116 comes from 81Br + 35Cl or 79Br + 37Cl
114 comes from 79Br + 35Cl