Average Atomic Mass
Chapter 7.4
Average Atomic Mass
• Objective:
(1) To calculate the average atomic mass for an
atom.
Average Atomic Mass
• Weighted average of the masses of the
existing isotopes of an element.
Recall: Atomic Mass Unit
• The unit that describes the mass of an
atom
• Symbol: amu
Example #1
• The mass of a Cu-63 atom is 62.94 amu, and
that of a Cu-65 atom is 64.93 amu. The
percent abundance of Cu-63 is 69.17% and
the percent abundance of Cu-65 is 30.83%.
What is the average atomic mass of Cu?
Example #1
• The mass of a Cu-63 atom is 62.94 amu, and that of a Cu-65
atom is 64.93 amu. The percent abundance of Cu-63 is 69.17%
and the percent abundance of Cu-65 is 30.83%. What is the
average atomic mass?
• Step 1: Find the contribution of each isotope:
Cu-63: (62.94 amu) x (0.6917) = 43.535598 amu
Cu-65: (64.93 amu) x (0.3083) = 20.017919 amu
• Step 2: Add the relative abundances from each isotope together.
43.535598 amu + 20.017919 amu = 63.553517 amu
Round answer to two numbers after the decimal: 63.55 amu
Example #2
• Calculate the average atomic mass of
chromium. It is made up of isotopes with
the following percent compositions and
atomic masses: 83.79% with a mass of 51.94
amu; 9.50% with a mass of 52.94 amu;
4.35% with a mass of 49.95 amu; 2.36 %
with a mass of 53.94 amu.
Example #2
• Calculate the average atomic mass of chromium. It is made up of
isotopes with the following percent compositions and atomic masses:
83.79% with a mass of 51.94 amu; 9.50% with a mass of 52.94 amu;
4.35% with a mass of 49.95 amu; 2.36 % with a mass of 53.94 amu.
• Step 1: Find the contribution of each isotope:
(51.94 amu) x (0.8379) = 43.520526 amu
(52.94 amu) x (0.0950) = 5.0293 amu
(49.95 amu) x (0.0435) = 2.172825 amu
(53.94 amu) x (0.0236) = 1.272984 amu
• Step 2: Add the relative abundances from each isotope together.
43.520526 amu + 5.0293 amu + 2.172825 amu + 1.272984 amu =
51.995635 amu
Round answer to two numbers after the decimal: 52.00 amu
Example #3
• Calculate the average atomic mass of iron if its
abundance in nature is 15% iron-55 and 85%
iron-56.
Example #3
• Calculate the average atomic mass of iron if its
abundance in nature is 15% iron-55 and 85%
iron-56.
• Step 1: Calculate the contribution of each
isotope:
Iron-55: (55 amu) x (0.15) = 8.25 amu
Iron-56: (56 amu) x (0.85) = 47.6 amu
• Step 2: Add the contribution from each
isotope together:
8.25 amu + 47.6 amu = 55.85 amu
Example #4
• Chlorine exists as chlorine-35, which has
a mass of 34.969 amu and makes up
75.8% of chlorine atoms. The rest of
naturally occurring chlorine is chlorine37, with a mass of 36.996 amu. What is
the average atomic mass of chlorine?
Example #4
• Chlorine exists as chlorine-35, which has a mass of 34.969 amu
and makes up 75.8% of chlorine atoms. The rest of naturally
occurring chlorine is chlorine-37, with a mass of 36.996 amu.
What is the average atomic mass of chlorine?
• Step 1: Find the contribution of each isotope:
Cl-35: (34.969 amu) x (0.758) = 26.506502 amu
Cl-37: (36.996 amu) x (0.242) = 8.953032 amu
• Step 2: Add the relative abundances from each isotope together.
26.506502 amu + 8.953032 amu = 35.459534 amu
Round answer to two numbers after the decimal: 35.46 amu
Example #5
• Element X has two isotopes. One has a mass of
10.0 amu and an abundance of 20.0%. The other
has a mass of 11.0 amu and an abundance of
80.0%. Estimate the average atomic mass. What
element is it?
Example #5
• Element X has two isotopes. One has a mass of 10.0 amu
and an abundance of 20.0%. The other has a mass of 11.0
amu and an abundance of 80.0%. Estimate the average
atomic mass. What element is it?
• Step 1: Calculate the contribution of each isotope:
Isotope 1: (10.0 amu) x (0.20) = 2 amu
Isotope 2: (11.0 amu) x (0.80) = 8.8 amu
• Step 2: Add the contribution from each isotope together:
2 amu + 8.8 amu = 10.8 amu
The element with an atomic mass of 10.8 amu is Boron.