Question

The atomic weight of a newly discovered element is 98.225 amu. it has two naturally occuring isotopes. one isotope has a mass of 96.780 amu. the second isotope has a percent abundance of 41.7%. what is the mass of the second isotope?

Answer 1

Atomic weight of an element can be calculated as follows:
average atomic weight = 
(atomic weight of first isotope)(its percentage of abundance) + 
(atomic weight of second isotope)(its percentage of abundance)

average atomic weight = 98.225 amu
atomic weight of first isotope = 97.780 amu
its percentage of abundance = 1 - 0.417 = 0.583
atomic weight of second isotope = ??
its percentage of abundance = 0.417

So, just substitute in the above equation to get the atomic weight of the second isotope as follows:
98.225 = (97.78)(0.583) + (mass2)(0.417) 
atomic weight of second isotope = 98.847 amu