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 ExampleA box contains two size of marbles. If25.0% have masses of 2.00 g and 75.0%have masses of 3.00 g what is the weightedaverage?(.250) (2.00) + (.750) (3.00) = =2.75g
3 Calculating Average Atomic Mass EXAMPLEBoron has two isotopes:B-10 (mass amu) 19.8% abundanceB-11 (mass amu) 80.2% abundanceCalculate 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 MassHelium 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 penniesb. Let 10-X be the number of post-1982 penniesc. (X)(3.1g) + (10-X)(2.5g) = mass of 10 penniespre post-82EXAMPLE (Mass of a sample of pennies is 31.0g)(X)(3.1g) + [(10-X)(2.5g)] = 31.0 g3.1X X = 31.0g.6X + 25 = 31.0g.6X = 6.0g X = 6.0g/.6X = 10 pre-82 pennies10-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