Calculations in Chemistry:

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# Calculations in Chemistry: - PowerPoint PPT Presentation

Calculations in Chemistry:. Average Atomic Mass. Isotopes. An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.

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### Calculations in Chemistry:

Average Atomic Mass

Isotopes
• An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.

For example, there are two different types (isotopes) of copper atoms. One type of copper atoms weighs in at 62.93 amu, the other has a mass of 64.94 amu.

• These two isotopes have different proportions in a natural sample of copper.
• The lighter isotope is more common with  69.09% of the naturally occurring copper having a mass of 62.93 amu per atom.
• What is the percent abundance of the other isotope?
Average Atomic Mass
• To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type.
•  The calculation of the average atomic mass is a WEIGHTED AVERAGE.
Calculation of Average Mass

Average atomic mass =

Σ (mass of isotope × relative abundance)

Σ = sum or to add them all together

Calculation of Average Mass
• The bottom line is that to find the average atomic mass of copper, we insert the information about copper’s isotopes into the formula and solve.
• There are two isotopes, so we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in. )
• The relative abundance is simply the percentage of the isotope, but in decimal format.  69.09% corresponds to a relative abundance of 0.6909.
Calculation of Average Mass

Average atomic mass of copper =

(62.93 amu × 0.6909)

[the mass and abundance of isotope #1]

+

(64.94 amu × 0.3091)

[the mass and abundance of isotope #2]

=

63.55

Calculation of Average Mass
• From the calculation, we know that an AVERAGE atom of copper has a mass of 63.55 amu.
• Notice that in this problem, we would predict that the average is closer to the weight of the lighter isotope.
• This is because the lighter form of copper is more abundant.