Theatomic mass of copper is 63.55.There are only two isotopes of copper , Cu-63 with a mass of 62.93 amu and Cu-65 w/ a mass of 64.93 amu. What is the percent of each of these two isotopes?

Answer 1

Total is 100%, so assuming x is the fraction of Cu-63, (1-x) is the fraction of Cu-65.

Atomic weight is the (weighted) average of all isotopes.

Therefore you have:

x*62.93 + (1-x)*64.93 = 63.55
x = 0.69.

So you have 69% Cu-63 and 31% Cu-65.