Key Concepts: Calculating Empirical Formulas

Empirical formulas represent the simplest whole-number ratio of atoms in a compound.
An empirical formula may not always signify the actual number of atoms in a molecule, but it always represents the ratio in simplest terms.
To calculate a compound’s empirical formula, you must know the mass or percentage of each element present.
First, if given percentages, consider them as masses. For instance, think of 40% oxygen as 40g of oxygen. This is because the ratios of masses are equivalent to the ratios of percentages.
Next, convert mass to moles by dividing each element’s mass by its atomic mass. This information can be found on the periodic table where the atomic mass is usually listed below the element symbol.
For each element, divide the number of moles by the smallest number of moles you calculated. This gives the simplest, whole-number ratio of atoms.
If you don’t end up with whole numbers, you may need to multiply all the numbers by the same factor to get whole numbers. This is usually 2, but sometimes can be 3 or more.
The numbers you end up with are the subscript numbers in the empirical formula. For example, in the formula CH2, there is one atom of carbon (C) and two atoms of hydrogen (H).
Remember to check your work! Balancing subscripts can be tricky, so verify that the proportions of atoms match the original information given for the compound.
Empirical formulas are commonly used in chemistry, especially for ionic compounds where the individual molecules do not exist separately.