Answer :
The average atomic mass of lanthanum that has an isotope with an atomic mass of 138.9 amu and an abundance of 99.9 1% and a second isotope with an atomic mass of 137.9 amu and an abundance of 0.09 % is
Average atomic m = Sum of product of atomic m and % abundance / 100
For isotope 1,
Atomic mass = 138.9 amu
% abundance = 99.91 %
For isotope 2,
Atomic mass = 137.9 amu
% abundance = 0.09 %
Average atomic mass = ( 138.9 * 99.91 ) + ( 137.9 * 0.09 ) / 100
Average atomic mass = 13877.5 + 12.41 / 100
Average atomic mass = 138.9 amu
The average atomic mass or average atomic weight of an element is the weighted average mass or weight of atoms in a naturally occurring sample ( i.e., isotopes ) of the element.
Therefore, the average atomic mass of lanthanum is 138.9 amu
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