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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 12 C was assigned an exact mass of 12.00 amu Relative masses of all other atoms was determined by comparing each to the mass of 12 C 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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Listed on the periodic table Gives the mass of “average” atom of each element compared to 12 C Average atom based on all the isotopes and their abundance % Atomic mass is not a whole number Na 22.99 Atomic Mass

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4 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 isotope 1 (%) + mass isotope 2 (%) + … 100 100 Calculating Atomic Mass

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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?

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