Assume there are only two isotopes present. The average atomic mass is 69.7. If 60.4% is isotope Ga69, then 39.6% has a different mass, M. Solve this weighted-average-mass equation for M:
69.7 = (.604)(68.93) + (.396)M
.396M = 69.7 - 41.63 = 28.07
M = 70.87
That other isotope would probably be isotope Ga71.
A table of isotope abundances confirms this result
Gallium has an atomic mass of 69.7 amu. In a typical sample, 60.4% of Ga exxists as a Ga-69 (68.9257 amu). What isa the identity and the atomic mass of the other isotope?
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