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**Average Atomic Mass & % Abundance**

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Average Atomic Mass The weighted average of the atomic masses of the naturally occurring isotopes of an element Most elements occur naturally as mixtures of isotopes

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Average Atomic Mass Dependent upon both mass and the relative abundance of each of the elements isotopes

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Example Naturally occurring copper exists with the following abundances: 69.17% is Cu-63 w/ atomic mass amu 30.83% is Cu-65 w/ atomic mass 64.93 (.6917)x(62.93) + (.3083)x(64.93)= amu

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**Problem 1 3 Isotopes of Ar occur in nature 0.337% as Ar-36, 35.97 amu**

Calculate the Average Atomic Mass

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Answer Check (.00337)x(35.97) + (.00063)x(37.96) + (.996)x(39.96)= 39.95amu

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Problem 2 2 Naturally occurring Isotopes of Boron occur with the following abundances: 80.20% B-11, amu 19.80% B-10, amu What is the Average Atomic Mass

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Answer Check (.8020)x(11.01) + (.1980)x(10.81) = amu

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**Calculating & Abundance**

Chlorine has two isotopes: chlorine-35 (mass amu) and chlorine-37 (mass amu). What is the percent abundance of these two isotopes if chlorine's atomic mass is ?

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Answer Check Part 1 if 2 isotopes, then the total is 100%. assume one is x% (x), the other is automatically 100-x%, (1-x) x(34.97) + (1-x)(36.97) =

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**Answer Check Part 2 x(34.97) + (1-x)(36.97)=35.453 Solve for x**

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Answer Check Part 3 Therefore Cl-35 has a % abundance of 75.85% and Cl-37 has a % abundance of 24.15%

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Problem 1 The two naturally occurring isotopes of nitrogen are nitrogen-14, with an atomic mass of amu, and nitrogen-15, with an atomic mass of amu. What are the percent natural abundances of these isotopes? The atomic mass of nitrogen is amu

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**Answer Check The atomic mass of nitrogen is 14.00674amu**

= p( ) + (1 -p)( ) = p p p = p = = 99.63% (N14) 1 - p = = 0.37% (N15)

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