when we first began to analyze the composition of various substances we were able to find the ratios of the different elements in a compound by mass and therefore by number of atoms by taking for example an unknown hydrocarbon and combusting it we can collect the carbon dioxide and water that results to determine the original masses of carbon and hydrogen in the hydrocarbon in order to find the ratio of the two elements in the compound perhaps we analyze a hydrocarbon and find that there are two hydrogen atoms for every carbon atom we could then say that the empirical formula for that hydrocarbon would be ch2 that's because the empirical formula of a compound lists the lowest whole number ratio of all the elements in the compound however an empirical formula does not tell us precisely how many atoms of each element are in a molecule the number of atoms that are actually in a molecule is listed in a molecular formula let's see how many molecules we can come up with that have the empirical formula ch2 there aren't really any with just one carbon and two hydrogens but this empirical formula will also include molecules with precisely two carbon atoms and four hydrogen atoms like this or six carbon atoms and 12 hydrogens like this this is because a molecule with a molecular formula of c6h12 would still have an empirical formula of ch2 since the empirical formula gives the lowest whole number ratio for the atoms of each element in the compound let's see if we can assign empirical and molecular formulas for each of these compounds list both types and see if any of them have an empirical formula that is the same as the molecular formula these are the answers we should have gotten and now we know a bit about empirical and molecular formulas