Finding Empirical and Molecular Formula from Percent Composition
% composition = (element mass / compound mass) X 100
If you are given the % composition of a compound, here are the steps for finding the empirical formula:
1. Assume you have a 100 g sample. This makes the calculation simple because the
percentages will be the same as the number of grams. For example, if 40% of the mass of a
compound is oxygen then you calculate you have 40 grams of oxygen.
2. Convert grams to moles. Empirical formula is a comparison of the number of moles of a
compound so you need your values in moles. Using the oxygen example again, there are 16.0
grams per mole of oxygen so 40 grams of oxygen would be 40/16 = 2.5 moles of oxygen.
GRAMS ---> MOLES divide the mass given to you by molar mass
THEN YOU DIVIDE BY THE SMALLEST NUMBER
EX .857g H and 6.86 g of O
0.857 g H x (1 mol H/ 1.01 g H) = 0.849 mol H / 0.429 mol = 1.98 6.86 g O x ( 1 mol O / 16 g O) = 0.429
mol O / 0.429 mol = 1
3. Compare the number of moles of each element to the smallest number of moles you
got and divide by the smallest number.
4. Round your ratio of moles to the nearest whole number as long as it is close to a whole
number. In other words, y