Online
Introductory Chemistry
Isotopes and average
atomic masses
- Example
weighted average calculation using isotope
abundances:
What is the average atomic weight for chlorine if it
has two isotopes? The percent abundance for chlorine-35
is 75.53%. The percent abundance for chlorine-37
is 24.4%. The mass for Cl-35 is 35.0 amu and for Cl-37 it
is 37.0 amu.
- Average
atomic weight = 26.4 amu + 9.05 amu = 35.5
amu
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- Notice
that the average is determined by the more
abundant Cl-35. Chlorine-35 is more common so
the average is closer to 35 amu than 37 amu.
Clearly no atoms of thallium actually have a
mass of 35.5 amu. The tabulated value in the
periodic table is a statistical
creation.
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- Exercise:
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What
is the average atomic mass for thallium, Tl, if
there are two isotopes with the following masses
and abundances?
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Tl-203 has a mass of 203.059 amu with an abundance
of 29.52 %
Tl-205
has a mass of 205.059 amu with an abundance of 70.48
%
- You
need to show your work. Answer: 204.466 amu rounded off
to four significant figures is 204.46 amu
- Click
here to see a detailed solution.
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