Question

Can someone help please?

Copper occurs naturally as a mix of two isotopes: Copper-63 with a mass of 62.930 amu and copper-65 with a mass of 64.928 amu. The percent abundance of copper-63 is 69.09%.
Calculate the atomic mass of copper.

Answer 1

Take the weighted average of the individual isotopes.

Step-by-step explanation:

63

C

u

has

69.2

%

abundance.

65

C

u

has

30.8

%

abundance.

So, the weighted average is

62.93

×

69.2

%

+

64.93

×

30.8

%

=

63.55

amu

.

If we look at the Periodic Table, copper metal (a mixture of isotopes but

63

C

u

and

65

C

u

predominate) has an approximate atomic mass of

63.55

g

⋅

m

o

l

−

1

, so we know we are right.

Answer 2

Answer: 63.55

Step-by-step explanation:

The attached chart shows the calculation. The orange line points to the % abundance of Cu-65, which is the difference of 100% less the % Abundance of Cu-65 (since there are only 2 isotopes).

The weighted average contribution of each istope is calculated (blue arrow) and then summed to find the atomic mass of copper.