When calculating percent abundance for a group, you need to know the total number of atoms in the group and the number of atoms of the particle isotopes in the problem. Use your numbers to
1. write the fraction. (The isotope is x atoms out of the total number of atoms)
2. Convert the fraction to a decimal by dividing the total number into the number of atoms of the isotope. If you do this correctly, the decimal will always be less than 1.
3. Multiply by 100. (move the decimal point to the right two places)
** Why do we multiply by 100? Because percent (%) stands for x out of a group of 100. When we are calculating percent abundance we are converting from knowing to how many of a particular isotope are in whatever random number of atoms is in the picture to predicting how many would be in a group of 100.
When calculating the average mass of a group, you need to know the mass of all the atoms (total mass) and the number of atoms. The average mass is the total mass divided by the number of atoms. I find it easiest to
1. Calculate the mass of each isotope (number of atoms times the isotope mass)
2. Add up all the mass of all the different isotopes
3. Divide by the total number of atoms