Question

Indium (In ) has two naturally occurring isotopes: In−113 and In−115 . In−113 has a 4.30 % abundance and a mass of 112.9 amu , and In−115 has a 95.70 % abundance and a mass of 114.9 amu .

Part A

Calculate the atomic mass for indium using the weighted average mass method.

Answer 1

The average atomic mass is the sum of masses of isotopes, each multiplied by their natural abundance.

So for indium ,

Atomic mass = ( 4.30 × 112.9) / 100 + (95.70 × 114.9) / 100

= 114.8 amu

So the atomic mass of indium is 114.8 amu