In order to find an empirical formula of a compound with a given percent composition of the elements that compose this molecule, we need so start by assuming an initial mass of 100 grams (it can be any value, but we usually choose 100 grams because it is easier to work on)
Now, if this compound has 100 grams, we can find the mass of each element and with that information, we can find the number of moles of each one
