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Calculating Average Atomic Mass. Introduction. The value for atomic mass for a particular element is the average atomic mass. Each sample of an element is composed of a mixture of different isotopes. Therefore, we need to average the masses of the isotopes to find the average atomic mass. - PowerPoint PPT Presentation
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Calculating Average
Atomic Mass
Introduction• The value for atomic mass for a particular
element is the average atomic mass.• Each sample of an element is composed of a
mixture of different isotopes.• Therefore, we need to average the masses of the
isotopes to find the average atomic mass.
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• We would expect the average atomic mass to be somewhere in between 63 and 65.
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages (as a
decimal value)
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages (as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages (as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages (as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages (as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
o and add the values
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages(as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
o and add the values• 43.56 + 20.05 =
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages(as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
o and add the values• 43.56 + 20.05 =
Calculations• Copper has two common isotopes.
o 63Cu is 69.15% of copper samples.o 65Cu is 30.85% of copper samples.
• To find the average atomic masso multiply the mass number by the percentages(as a
decimal value)• 63 × 0.6915 = 43.56• 65 × 0.3085 = 20.05
o and add the values• 43.56 + 20.05 = 63.61
Calculations• Iron has three common isotopes.
o 54Fe is 5.85% of iron samples.o 56Fe is 91.75% of iron samples.o 58Fe is 2.82% of iron samples.
• We would expect the average atomic mass to be somewhere in between 54 and 58.
Calculations• Iron has three common isotopes.
o 54Fe is 5.85% of iron samples.o 56Fe is 91.75% of iron samples.o 58Fe is 2.82% of iron samples.
• multiply the mass number by the percentageso 54 × 0.0585 = 3.16o 56 × 0.9175 = 51.38o 58 × 0.0282 = 1.61
Calculations• Iron has three common isotopes.
o 54Fe is 5.85% of iron samples.o 56Fe is 91.75% of iron samples.o 58Fe is 2.82% of iron samples.
• multiply the mass number by the percentageso 54 × 0.0585 = 3.16o 56 × 0.9175 = 51.38o 58 × 0.0282 = 1.61
• and add the values
Calculations• Iron has three common isotopes.
o 54Fe is 5.85% of iron samples.o 56Fe is 91.75% of iron samples.o 58Fe is 2.82% of iron samples.
• multiply the mass number by the percentageso 54 × 0.0585 = 3.16o 56 × 0.9175 = 51.38o 58 × 0.0282 = 1.61
• and add the valueso 3.16 + 51.38 + 1.61 = 56.15
Try one on your own• Magnesium has three common isotopes.
o 24Mg is 78.99% of magnesium samples.o 25Mg is 10.00% of magnesium samples.o 26Mg is 11.01% of magnesium samples.
• What would be the average atomic mass of Mg?
Do not move on until you try this problem.
Try one on your own• Magnesium has three common isotopes.
o 24Mg is 78.99% of magnesium samples.o 25Mg is 10.00% of magnesium samples.o 26Mg is 11.01% of magnesium samples.
• What would be the average atomic mass of Mg?o 24 × 0.7899 = 18.96o 25 × 0.1000 = 2.50o 26 × 0.1101= 2.86
Try one on your own• Magnesium has three common isotopes.
o 24Mg is 78.99% of magnesium samples.o 25Mg is 10.00% of magnesium samples.o 26Mg is 11.01% of magnesium samples.
• What would be the average atomic mass of Mg?o 24 × 0.7899 = 18.96o 25 × 0.1000 = 2.50o 26 × 0.1101= 2.86
o 18.96 + 2.50 + 2.86 = 24.32
Try one on your own• Chlorine has two common isotopes.
o 35Cl is 75.76% of chlorine samples.o 37Clg is 24.24% of chlorine samples.
• What would be the average atomic mass of Cl?
Do not move on until you try this problem.
Try one on your own• Chlorine has two common isotopes.
o 35Cl is 75.76% of chlorine samples.o 37Clg is 24.24% of chlorine samples.
• What would be the average atomic mass of Cl?o 35 × 0.7576 = 26.52o 37 × 0.2424 = 8.97
Try one on your own• Chlorine has two common isotopes.
o 35Cl is 75.76% of chlorine samples.o 37Clg is 24.24% of chlorine samples.
• What would be the average atomic mass of Cl?o 35 × 0.7576 = 26.52o 37 × 0.2424 = 8.97
o 26.52 + 8.97 = 35.48
Summary• To find the average atomic mass of an element,
we need to know the percent abundance of each isotope.
• We multiply the mass number of each isotope by the percentages (as a decimal number) and add the values for each isotope to get the average atomic mass.
