Question

The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are:

69.2% for a mass of 63
30.8% for a mass of 65.

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.