Question

Naturally occurring copper exists in two isotopic forms. 63Cu and 65Cu. The atomic mass of copper is 63.55 amu. What is the approximate natural abundance of 63Cu?

Answer 1

It's a weighted average.

Atomic mass = (abundance of 63Cu* mass of 63Cu)+ (abundance of 65Cu* mass of 65Cu)

Answer 2

But how do you find the abundance? All I was given was in the question.

Answer 3

Use algebra.
\[63x+65y=63.55\]
\[x+y=1\]

It would be 1 and not 100 because natural abundance is in percentage.
80% would be 0.80
and 70% would be 0.70
Natural abundance of both isotopes would need to be at 100% since those are the only isotopes for copper. 100%=1

Answer 4

There is another method but I forgot.

Answer 5

Alright. Thanks.

Answer 6

No problem!

But what did you get for each?

Answer 7

I got 63 Cu = 62.925989 and 65 Cu = 64.9277929

Answer 8

Then I did all the math and got x=.689

Answer 9

Sorry about the late replies.

Answer 10

63Cu = 68.95%
65 Cu = 31.05%

Answer 11

You did the math wrong.
63(0.6895)+65(0.3105) did not equal 63.55.
It equaled 63.62 so your answer is a bit close to the real one.
You can check your answers this way.

Answer 12

I'll have to go back and see what I did wrong. I guess my professor is wrong too. lol I got the same answer as them. Probably should let them know.

Answer 13

They are wrong because my answers matches to 63.55 amu.

Answer 14

I'll let them know. Thanks for checking the math for me.

Answer 15

I'm bad about making little mistakes in the math. It can really throw it off.

Answer 16

No problem!