How to calculate the relative abundance of an isotope

We can calculate the percentage abundances for each of the isotopes in an element if we know the mass number for each isotope and the A_r. This is another of those situations where the maths is really quite complex, unless you study Maths A level, and it is not covered in the textbooks … so make some notes and build your confidence with the practice questions 😊.

The best way to get your head around this is with a worked example.

e.g. Iridium has 2 isotopes, ¹⁹¹Ir and ¹⁹³Ir.  Given that the A_r of iridium is 192.2, calculate the % abundance of each isotope.

Practice questions

1. Determine the % abundance of the two isotopes of lithium, ⁷Li and ⁶Li, given that the relative atomic mass of the element is 6.94.

2. Silver has two isotopes, ¹⁰⁷Ag and ¹⁰⁹Ag. Calculate the % abundance of each given that silver’s A_r is 107.9.

3. The relative atomic mass of antimony is 121.8 and the element consists of two isotopes, ¹²¹Sb and ¹²³Sb. Determine the relative abundance of each isotope. 

4. A sample of magnesium has an Ar of 24.31 and consists of three isotopes, two of which are ²⁴Mg (with a relative abundance of 78.99%) and ²⁵Mg (with a relative abundance of 10.00%).  Calculate the relative abundance and the mass number for the third isotope, showing your working. 

Answers