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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.

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Calculations in chemistry

Calculations in Chemistry:

Average Atomic Mass


Isotopes
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
Average Atomic Mass copper atoms. One type of copper atoms weighs in at 

  • 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
Calculation of Average Mass copper atoms. One type of copper atoms weighs in at 

Average atomic mass =

 Σ (mass of isotope × relative abundance)

Σ = sum or to add them all together


Calculation of average mass1
Calculation of Average Mass copper atoms. One type of copper atoms weighs in at 

  • 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 mass2
Calculation of Average Mass copper atoms. One type of copper atoms weighs in at 

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 mass3
Calculation of Average Mass copper atoms. One type of copper atoms weighs in at 

  • 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.


Practice
Practice copper atoms. One type of copper atoms weighs in at 


Check your answers
Check Your Answers copper atoms. One type of copper atoms weighs in at 

  • Silver = 107.87 amu

  • Silicon = 28.09 amu

  • Iron = 55.84 amu


Homework
Homework copper atoms. One type of copper atoms weighs in at 

  • Read section 1.4 in the text to support what we did in class today.

  • pg27 #1

  • Pg 29 #1-5,9


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