[methlover]

Q: 69% of naturally occurring copper is copper-63.  If only one other isotope is present in natural copper, what is it?

choices:
A)copper-61
B)copper-62
C)copper-64
D)copper-65

Here is how I tried to solve this problem.  I looked at the periodic table, and the atomic weight for copper is 63.5g/mol.  So I can eliminate choice A and B.  Now I am left with copper-64 and copper-65.  Since copper 64 is close to 63.5g/mol than copper 65, isn't copper-64 the other isotope present in natural copper??

I don't even remember learning this in gen. chem.  Someone try to explain why it's copper-65 instead of copper-64 because the book's explanation didn't help me much.

[Borek]

Think how you can calculate molar mass knowing isotopes percentages.

[ATMyller]

Someone try to explain why it's copper-65 instead of copper-64 because the book's explanation didn't help me much.

Copper-64 is radioactive with 12.7 hour half live so there is none left in natural deposits. And if the remaining 31% would be copper-64 the average atomic weight would be less than 63.5 g/mol.