Question

An unknown element has two isotopes. The first isotope has a mass of 68.926 amu and an abundance of 60.108%. The second isotope has a mass of 70,925 amu and an abundance
of 39.892%. Calculate the average atomic mass AND identify the element.

Answer 1

M
r
(
G
a
)
=
69.7231
to 4 decimal places
.
Explanation:
To find the average atomic mass (
A
r
) of an element, given the percentage rarities of its isotopes, simply multiply each isotope's
A
r
by its percentage that is expressed as a decimal and total the results. We can carry this out in a single line of working:
M
r
(
Ga
)
=
(
68.9256
⋅
0.60108
)
+
(
70.9247
⋅
0.39892
)

M
r
(
Ga
)
=
69.7231
to 4 decimal places

Similarly, you could input the percentages normally and then divide by
100
:
M
r
(
Ga
)
=
(
68.9256
⋅
60.108
)
+
(
70.9247
⋅
39.892
)
100

M
r
(
Ga
)
=
69.7231
to 4 decimal places

Notice that the answers are the same. Also observe that the answer is closer to 69 than it is to 71, which makes sense since gallium-69 makes up a greater proportion of gallium in the universe (
~
60
%
) than gallium-71 does (
~
30
%
)

Answer 2

Answer:

69.72344108

Gallium

Explanation: