Determination of empirical formulas

The empirical formula is a formula that shows the atoms forming a compound but not the actual numbers or arrangement of atoms.

For Determining the empirical formula using percent composition:

1- We use experimental analysis to tell us the number of grams of each element present in a given amount of a compound.

2- We convert the quantities in grams to the number of moles of each element.

3- We use number of moles to find the empirical formula of the compound.

For example, let us consider a compound that we need to know its empirical formula. If the compound is burned, carbon dioxide (CO₂) and water (H₂O) are given off.

We can conclude both carbon (C) and hydrogen (H) percent in ethanol from the masses CO₂ and of H₂O. Because oxygen was used to burn methanol we will deduce oxygen in the sample by subtracting hydrogen and carbon from the original sample.  The masses of CO₂ and of H₂O can be concluded from the difference in CO₂ and of H₂O absorbers. If in one experiment 22.0g of ethanol produced 22.0g of CO₂ and 13.5g of H₂O.

Suppose that in the combustion of 11.5 g of material produced 22.0 g of CO₂ and 13.5 g of H₂O: 

Before we can predict the empirical formula we need to calculate the number of moles:

So the empirical formula of this compound is C₂H₆O. Still, we do not know how the atoms are linked together in the compound.