# Silver has two naturally occurring isotopes. Ag- 107 with a mass of 106.905 amu and a natural abundance of 51.84 % and Ag-109. How do you use the atomic mass of silver listed in the periodic table to determine the mass of Ag-109?

##### 1 Answer

Using my (not very precise) periodic table, I get the mass of Ag as 107.87.

This must be a combination of these two isotopes. So the % of the other isotope must be 48.16%.

To work with these percentages in an equation I'll use them as decimals.

So now I can start putting them into an equation:

Because both of the isotope masses, multiplied by their percentage, would give us the overall average mass.

Worked out the brackets that we can work out, and then rearrange to give:

Again rearrange:

And final rearranging:

You probably want to work through that again using your periodic table value for Ag.