[jeBK]

Hello!  my name is Jenora and I have a chemistry question for you.
Naturally occurring copper,Cu, is composed of two isotopes.  The isotope copper-63 is 69.17% of naturally occurring copper and has an atomic mass of 62.94 amu.  The isotope copper-65 has an atomic mass of 64.93 amu.  What percent  of naturally occurring copper is copper-65?

[hmx9123]

Copper's weight on the periodic table is a mixture of all of the stable isotopes.  Specifically, it's a weighted average.

Specifically, the list weight for Cu is 63.546 amu.  This number is figured by multiplying the weights of the isotopes by their % occurrance in nature, thus making a weighted average.  For copper:

63.546 amu = (Isotope 1 x % abundance) + (Isotope 2 + % abundance)

You are given the masses of both isotopes and the % abundance of one.  Your only need is to look up the weighted average of their weights, which is the mass of Cu on the periodic table and is already mentioned above.  Filling in, your equation is this:

63.546 amu = (62.94 amu x 69.17%) + (64.93 amu x ? %)

Solving for the ?, we get: 30.82%.  Now, the stupid thing about this problem is that since you know that there's only two isotopes of copper, you could just subtract 69.17% from 100% and get 30.83%, which is within rounding errors.  This question would have been a lot more interesting if they had asked you for the weight of copper 65.  With just the mass and % abundance of the copper 63 isotope, you could solve the whole problem.

[AWK]

Hmx9123 is absolutely right in his(her) comment concerning weighted average of atomic mass.

 But question was: "What percent  of naturally occurring copper is copper-65" . and percentage of the second isotope for two-components mixture was done.
For such a case always sum of percentage for both component should be equal 100.