Copper occurs in an isotopic mixture of 69.09% 63Cu (mass = 62.93 u per atom) and 30.91% 65Cu (mass = 64.93 u per atom). What is the average atomic mass of copper?

Question

Quizplus · Accepted Answer

To find the average atomic mass, we need to multiply the mass of each isotope by its percent abundance, add them together and then divide by 100.

average atomic mass = (0.6909 x 62.93 u) + (0.3091 x 64.93 u)
                             = 43.478017 u + 20.069763 u 
                             = 63.54778 u

Rounding to the nearest hundredth, the answer is choice letter B) 63.55 u.

Copper Occurs in an Isotopic Mixture of 69