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Average Atomic Mass

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Average Atomic Mass 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 ... – PowerPoint PPT presentation

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Title: Average Atomic Mass


1
Average Atomic Mass
2
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.

3
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

4
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

5
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.

6
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?

7
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.

8
Example 3
  • Calculate the average atomic mass of iron if its
    abundance in nature is 15 iron-55 and 85
    iron-56.

9
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?

10
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?
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