AVERAGE ATOMIC MASS CALCULATIONS

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AVERAGE ATOMIC MASS CALCULATIONS

Composition of the nucleus AVERAGE Atomic Masses The average of all the naturally occurring isotopes of that element. Isotope Symbol Composition of the nucleus % in nature Carbon-12 12C 6 protons 6 neutrons 98.89% Carbon-13 13C 7 neutrons 1.11% Carbon-14 14C 8 neutrons <0.01% Carbon = 12.011

WEIGHTED AVERAGE We want to take into account how much of each isotope there is so we get an accurate picture of what we are likely to “scoop up” Carbon = 12.011

Which is the most common isotope of carbon? WEIGHTED AVERAGE Average should give you a clue as to what the most common isotope is: Average mass of Carbon = 12.011 Which is the most common isotope of carbon? Carbon-12 Carbon-13 Carbon-14

Calculating average mass Avg. Mass = (MassIsotope1 x %abundance1 ) + (MassIsotope2 x %abundance2 ) + (MassIsotope3 x %abundance3 ) etc…

+ = 24.31 AVERAGE Atomic Masses X X X % (as a decimal) Isotope Mg-24 23.99 78.99%  0.7899 Mg-25 24.99 10.00%  0.1000 Mg-26 25.99 11.01%  0.1101 X 18.9497 X 2.499 + 2.861499 X = 24.31

Finding % Abundance Same equation, just solving for a different variable! We can use (x) to represent the %abundance1 We can use (1- x) to represent the %abundance2 BECAUSE: The total has to add up to 100% right?! 100% is the same as 1 to make the math faster

Finding % Abundance Boron has two naturally occurring isotopes: B-10 = 10.013amu B-11 = 11.009amu Avg Mass = (Mass1)(%abundance1) +(Mass2)(%abundance2) 10.81 = (10.013)(x) + (11.009)(1-x) solve for x B-10  x = 0.1998  *100 = 19.98% B-11  1-x = 1 - 0.1998 = 0.8002  *100 = 80.02% Calculate %abundance for each isotope if the average atomic mass of boron is 10.81amu