Beginner's Mass Ratio Problem

[Lorek]

1. The problem statement, all variables and given/known data

What is the ratio of mass of a 12C atom to a 13C atom

3. The attempt at a solution

12.011/13 = .9239230769

Solution given at back of book = .92286

[Borek]

Why 12.011? Why 13?

[Lorek]

The atomic weight of C-12 is 12.011, the atomic weight of C-13 is 13. I divided the weight of C-12 by the weight of the C-13 to get a ratio.

[Borek]

12.011 is not an atomic weight of C-12, just like 35.45 is not a mass of Cl atom.

[Lorek]

Ok, I understand that the atomic weight's are weighted averages. I just used the atomic weight because it asks for a ratio between two atoms. As for how to set it up I don't have the slightest idea how to set it up, no example problems to model form, nor solution steps.