Carbon has two naturally occurring isotopes, carbon-12 and carbon-13. The more common isotope of carbon is carbon-12 with an abundance of 98.93% while carbon-13 has an atomic mass of 13.00335 amu. Calculate the atomic mass of carbon-12 if the atomic mass of carbon is 12.0107 amu.
A)13.1 amu
B)13.0 amu
C)12.0 amu
D)12.8 amu
E)12.4 amu

Question

Quizplus · Accepted Answer

The atomic mass of carbon is the weighted average of the atomic masses of carbon-12 and carbon-13:
[0.9893 x 12.0000 amu (mass of carbon-12)] + [0.0107 x 13.00335 amu (mass of carbon-13)] = 12.0107 amu. 
Therefore, the atomic mass of carbon-12 (the more common isotope) is simply 12.0000 amu.

Carbon Has Two Naturally Occurring Isotopes, Carbon-12 and Carbon-13