Copper exists in two naturally occurring isotopes. the average atomic weight of copper is 63.546 amu. if 69.15% exists as 63cu with an atomic mass of 62.92, then what is the atomic mass of the second isotope?

Answer is: the atomic mass of the second isotope is 64.95 amu (Cu-65).

Ar(Cu) = 63.546 amu; the average atomic weight of copper.

Ar(Cu-63) =62.92; he average atomic weight of copper-63.

ω(Cu-63) = 69.15% ÷ 100% = 0.6915.

ω(Cu-second isotope) = 100% - 69.15%.

ω(Cu-second isotope) = 30.85 % ÷ 100% = 0.3085.

Ar(Cu) = Ar(Cu-63) · ω(Cu-63) + Ar(Cu-second isotope) · ω(Cu-second isotope).

63.546 amu = 62.92 amu · 0.6915 + Ar(Cu-second isotope) · 0.3085.

Ar(Cu-second isotope) · 0.3085 = 63.546 amu - (62.92 amu · 0.6915).

Ar(Cu-second isotope) · 0.3085 = 20.037.

Ar(Cu-second isotope) = 20.037 ÷ 0.3085.

Ar(Cu-second isotope) = 64.95.