# Average Atomic Mass & % Abundance

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Average Atomic Mass & % Abundance

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

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

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

Problem 1 3 Isotopes of Ar occur in nature 0.337% as Ar-36, 35.97 amu
Calculate the Average Atomic Mass

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

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

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

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 ?

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) =

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

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

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

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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