Calculating average atomic mass
A. The element copper has naturally occurring isotopes with
mass numbers of 63 and 65. The relative abundance and
atomic masses are 69.2% for mass = 62.93amu, and 30.8%
for mass = 64.93 amu. Calculate the average atomic mass of
copper.
B. Calculate the atomic mass of bromine. The 2 isotopes of
bromine have atomic masses and relative abundance of
78.92 amu (50.69%) and 80.92 amu (49.31%)
C. Neon has 2 isotopes. Neon-20 has a mass of 19.992 amu and
neon-22 has a mass of 21 amu. In any sample of 100 neon
atoms, 90 will be neon-20 and 10 will be neon-22. Calculate
the average atomic mass.
D. What is the atomic mass of silicon if 92.21% of its atoms
have mass 27.977 amu, 4.70% have mass 28.976, and 3.09%
have mass 29.974 amu?
Answers are on the next page.
ANSWERS.
There is more than one way to solve these problems. The solutions
below reflect this method. I did show the alternate way for problem
A only.
A.
.692 x 62.93 = 43.55
.308 x 64.92 = 20.00
63.55 amu
Another possible way to solve: (69.2 x 62.93) + (30.8 x 64.93) =
100
4354.76 + 1999.84 = 63.55
100
B.
.5069 x 78.92 = 40.00
.4931 x 80.92 = 39.90
79.90 amu
C
.90 x 19.992 = 17.99
.10 x 21.99 = 2.2
20.19 amu
D.
.9221 x 27.977 = 25.80
.0470 x 28.976 = 1.36
.0309 x 29.974 = 0.93
28.09
amu