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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 62.93 amu 30.83% is Cu-65 w/ atomic mass 64.93 (.6917)x(62.93) + (.3083)x(64.93)= 63.55 amu

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Problem 1 3 Isotopes of Ar occur in nature 0.337% as Ar-36, 35.97 amu 0.063% Ar-38, 37.96 amu 99.6% Ar-40, 39.96 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, 11.01 amu 19.80% B-10, 10.81 amu What is the Average Atomic Mass

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Answer Check (.8020)x(11.01) + (.1980)x(10.81) = 10.97 amu

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Calculating & Abundance Chlorine has two isotopes: chlorine-35 (mass 34.97 amu) and chlorine-37 (mass 36.97 amu). What is the percent abundance of these two isotopes if chlorine's atomic mass is 35.453?

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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) = 35.453

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Answer Check Part 2 x(34.97) + (1-x)(36.97)=35.453 Solve for x 34.97x+36.97-36.97x=35.453 -2x+36.97=35.453 -2x=-1.517 x=.7585 1-x=.2415

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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 14.003074 amu, and nitrogen-15, with an atomic mass of 15.000108 amu. What are the percent natural abundances of these isotopes? The atomic mass of nitrogen is 14.00674amu

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Answer Check The atomic mass of nitrogen is 14.00674amu 14.00674 = p(14.003074) + (1 -p)(15.000108) 14.00674 = 14.003074p + 15.000108 - 15.000108p - 0.997034p = -0.993368 p = 0.9963 = 99.63% (N14) 1 - p = 0.0037 = 0.37% (N15)

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