The empirical formula in chemistry provides the relative numbers of each type of atom in a particular molecule. It does not provide the exact number of each type of atom in the molecule, nor does it provide any information on the arrangement of those atoms. The empirical formula is used extensively in stoichiometry, a branch of analytical chemistry which studies the composition of reactants and products in chemical reactions. You can calculate the empirical formula of a compound from the amount of each element that is in a given sample of the compound.
Determine the mass of each element in a compound. These values are obtained experimentally, typically by conducting a series of reactions that break the compound down into its constituent elements. For this example, assume that we have 13.5 grams (g) of calcium (Ca), 10.8 g of oxygen (O) and 0.675 g of hydrogen (H).
Determine the number of grams in a mole (mol) of each element. This is given as the atomic weight of the element and is available from a periodic table. In this example, the atomic weight of Ca is 40.1, the atomic weight of O is 16.0 and the atomic weight of H is 1.01.
Calculate the number of moles of each element in the compound. For example, 13.5 g Ca / (40.1 g/mol Ca) = 0.337 mol Ca, 10.8 g O / (16.0 g/mol O) = 0.675 mol O and 0.675 g H / (1.01 g/mol H) = 0.668 mol H.
Determine the ratio of the elements in the compound. Divide the molar amount of each element by the smallest quantity. In this case, the smallest quantity is for calcium at 0.337 mol. By dividing each molar amount by 0.337 mol, we get 0.337/0.337 = one for calcium, 0.675/0.337 = about two for oxygen and 0.668/0.337 = about two for hydrogen.
Express the empirical formula for the sample. From Step 4, we know there are two atoms of oxygen and two atoms of hydrogen for each atom of calcium. The empirical formula for the sample compound is therefore CaO2H2.