Answer: 12.18 u
Explanation:
The average atomic mass of an element is calculated by taking the weighted average of the atomic masses of its stable isotopes.
In other words, each stable isotope will contribute to the average mass of the element proportionally to its abundance.
avg. atomic mass
=
∑
i
(
isotope
i
×
abundance
i
)
To calculate an isotope's decimal abundance, you need to divide the number of atoms of that isotope by the total number of atoms present in the sample.
In your case, you know that the sample contains
100
atoms with an atomic mass of
12.00 u
and
10
atoms with an atomic mass of
14.00 u
.
The total number of atoms in the sample will thus be
100
+
10
=
110 atoms
The decimal abundances for the two isotopes will be
100
atoms
110
atoms
=
0.90909
→
for the first isotope
10
atoms
110
atoms
=
0.090909
→
for the second isotope
This means that the average atomic mass of the element will be equal to
avg atomic mas
=
12.00 u
×
0.90909
+
14.00 u
×
0.090909
avg. atomic mass
=
12.18181 u
I think I'll leave this rounded off to four sig figs, the number of sig figs you have for the atomic masses of the two isotopes
avg. atomic mass
=
12.18 u
Hope this helps! :)
