This is our last step. We know the molar mass of each element and the whole compound. Now we need to divide each element mass over the combined total mass we got and multiply it by 100. This will give us the percent composition of each element in the compound.
Step 3: Determine Percent Compostion: Part/Whole x 100
So for Mg, we must do 24.3/95.3 and then multiply that by 100. Mg has a percent composition of about 25.5%. As for Cl, we do 71.0/95.3 and multiply it by 100 to get a percent composition of 74.5%
That's it! You have learned how to find the Percent Composition of elements in a compound!