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Calculations in Chemistry:

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:

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  1. Calculations in Chemistry: Average Atomic Mass

  2. 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.

  3. 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?

  4. The remainder of the atoms, 30.91 %, have a mass of 64.94 amu.

  5. 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.

  6. Calculation of Average Mass Average atomic mass =  Σ (mass of isotope × relative abundance) Σ = sum or to add them all together

  7. 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.

  8. 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

  9. 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.

  10. Practice

  11. Check Your Answers • Silver = 107.87 amu • Silicon = 28.09 amu • Iron = 55.84 amu

  12. Homework • 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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