Average Atomic Mass

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Average Atomic Mass
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Masses of Atoms

• A scale designed for atoms gives their small
  atomic masses in atomic mass units (amu)
• An atom of 12C was assigned an exact mass of
  12.00 amu
• Relative masses of all other atoms was
  determined by comparing each to the mass of 12C
• An atom twice as heavy has a mass of 24.00 amu.
  An atom half as heavy is 6.00 amu.

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Atomic Mass
Na 22.99

• Listed on the periodic table
• Gives the mass of average atom of each element
  compared to 12C
• Average atom based on all the isotopes and their
  abundance
• Atomic mass is not a whole number

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Calculating Atomic Mass

• Weighted average of the masses of the existing
  isotopes of an element.
• Percent() abundance of isotopes
• Mass of each isotope of that element
• Weighted average
• mass isotope1() mass isotope2()
• 100 100

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Sample Problem

• Calculate the average atomic mass of sulfur if
  95.00 of all sulfur atoms have a mass of 31.972
  amu, 0.76 has a mass of 32.971amu and 4.22 have
  a mass of 33.967amu.

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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?

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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.

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Example 3

• Calculate the average atomic mass of iron if its
  abundance in nature is 15 iron-55 and 85
  iron-56.

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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?

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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?