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What is average atomic mass?

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Presentation on theme: "What is average atomic mass?"— Presentation transcript:

1 What is average atomic mass?
Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element

2 Calculating a Weighted Average
Example A box contains two size of marbles. If 25.0% have masses of 2.00 g and 75.0% have masses of 3.00 g what is the weighted average? (.250) (2.00) + (.750) (3.00) = = 2.75g

3 Calculating Average Atomic Mass
EXAMPLE Boron has two isotopes: B-10 (mass amu) 19.8% abundance B-11 (mass amu) 80.2% abundance Calculate the average atomic mass. (.198) (10.013) + (.802) ( ) = 1.98 amu amu = amu

4 Calculating Average Atomic Mass
Calculate the average atomic mass of Mg. Isotope amu (78.99%) Isotope amu (10.00%) Isotope 3 – amu (11.01%) (23.985)(.7899)+(24.986)(.1000)+(25.982)(.1101) 18.95 amu amu amu = amu

5 Average Atomic Mass Helium has two naturally occurring isotopes, He-3 and He-4. The atomic mass of helium is amu. Which isotope is more abundant in nature? He-4 is more abundant in nature because the atomic mass is closer to the mass of He-4 than to the mass of He-3.

6 Isotopic Pennies – number of pre and post 1982
a. Let X be the number of pre-1982 pennies b. Let 10-X be the number of post-1982 pennies c. (X)(3.1g) + (10-X)(2.5g) = mass of 10 pennies pre post-82 EXAMPLE (Mass of a sample of pennies is 31.0g) (X)(3.1g) + [(10-X)(2.5g)] = 31.0 g 3.1X X = 31.0g .6X + 25 = 31.0g .6X = 6.0g X = 6.0g/.6 X = 10 pre-82 pennies 10-X = 0 pre-82 pennies

7 Isotopic Penny Lab- Average Atomic Mass
Calculate percent of pre-82 and post-82 pennies # of pre-82 pennies x 100% # post-82 pennies x 100% Calculate the average atomic mass of coinium (% pre-82)(3.1g) + (% post-82)(2.5) = average atomic mass

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