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 Ar. 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 Ar of iridium is 192.2, calculate the % abundance of each isotope.
Practice questions
- Determine the % abundance of the two isotopes of lithium, 7Li and 6Li, given that the relative atomic mass of the element is 6.94.
- Silver has two isotopes, 107Ag and 109Ag. Calculate the % abundance of each given that silver’s Ar is 107.9.
- 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.
- 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
