To calculate the percent abundance of each isotope in a sample of an element, chemists usually divide the number of atoms of a particular isotope by the total number of atoms of all isotopes of that element and then multiply the result by 100.
Herein, how do you calculate percent abundance from mass?
Use the atomic masses of each of the isotopes along with their percent abundances to calculate the average atomic mass. Change each percent abundance into decimal form by dividing by 100. Multiply this value by the atomic mass of that isotope. Add together for each isotope to get the average atomic mass.