Indium has only two naturally occurring isotopes. the mass of indium-113 is 112.9041 amu and the mass of indium-115 is 114.9039 amu. part a use the atomic mass of indium to calculate the relative abundance of indium-113.

The average atomic mass written in the periodic table is determined from the relative abundances of the element's isotopes. The equation would be:

Average Atomic Mass = ∑ (Relative Abundance*Mass)

Let be the relative abundance of Indium-113. Because there are only 2 isotopes, their relative abundances should equal to 1, such that the relative abundance for Indium-15 is (1-x). The atomic mass of indium is 114.818 amu.

x (112.9041) + (1 - x) (114.9039) = 114.818

Solving for x,

x = 0.043 or 4.3%