Question

the atomic weight of antimony is 121.76 amu. two naturally occurring isotopes of antimony. 121-Sb has an isotopic mass of 120.9038 amu and natural abundance of 57.40%. What is the isotopic mass of the other isotope, 123-Sb?

Answer 1

Isotope has 57.40% abundance
the other one 100% - 57.40% = 42.60%

mass of 121 (% abundance) + mass of 123 (% abundance) =  total mass (atomic weight)
(120.9038 amu)(0.5740) + (x amu)(0.426) = 121.76 amu
x = 122.9136

hope this help