What is average atomic mass?

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Presentation transcript:

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

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) = .500 + 2.25 = 2.75g

Calculating Average Atomic Mass EXAMPLE Boron has two isotopes: B-10 (mass 10.013 amu) 19.8% abundance B-11 (mass 11.009 amu) 80.2% abundance Calculate the average atomic mass. (.198) (10.013) + (.802) ( 11.009) = 1.98 amu + 8.83 amu = 10.81 amu

Calculating Average Atomic Mass Calculate the average atomic mass of Mg. Isotope 1 - 23.985 amu (78.99%) Isotope 2 - 24.986 amu (10.00%) Isotope 3 – 25.982 amu (11.01%) (23.985)(.7899)+(24.986)(.1000)+(25.982)(.1101) 18.95 amu + 2.499 amu + 2.861 amu = 24.31 amu

Average Atomic Mass Helium has two naturally occurring isotopes, He-3 and He-4. The atomic mass of helium is 4.003 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.

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-82 post-82 EXAMPLE (Mass of a sample of pennies is 31.0g) (X)(3.1g) + [(10-X)(2.5g)] = 31.0 g 3.1X + 25 - 2.5X = 31.0g .6X + 25 = 31.0g .6X = 6.0g X = 6.0g/.6 X = 10 pre-82 pennies 10-X = 0 pre-82 pennies

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% 10 10 Calculate the average atomic mass of coinium (% pre-82)(3.1g) + (% post-82)(2.5) = average atomic mass