now we know how to derive the empirical formula of a compound given either the mass of each element present in a sample of the compound or the percent composition of the compound while this is quite useful information we will also need to be able to derive molecular formulas these are more precise than empirical formulas because rather than simply stating the numerical ratio of the atoms of the different elements in the compound which results in many different possibilities the molecular formula will tell us exactly how many atoms of each element are in the compound with no ambiguity the way that we can derive the molecular formula is by first determining the molecular mass of the compound and then seeing how many units of the empirical formula it will take to equal that mass here's an example let's say we have an unknown substance and using the percent composition data we gather from an experiment we determine the empirical formula to be ch2o we can get the formula mass of the empirical formula by adding up the atomic masses of each atom 12 plus 2 plus 16 gives us 30 atomic mass units for the empirical formula mass let's say we also determine through various experiments that the molecular mass is 180 atomic mass units we can divide the molecular mass by the empirical formula mass to see how many formula units need to be combined to give the molecular formula 180 over 30 equals six so we will need six formula units to make one molecule of this particular compound let's take ch2o and multiply everything by six that will give us c6h12o6 this must be the molecular formula because it is the only option that obeys the numerical ratios given by the empirical formula and also adds up to give a molecular mass of 180 we have successfully determined the molecular formula of the compound you