So the way you basically go from a molecular to an empirical formula is you find a common number and you divide it by that common number, making sure that we get whole numbers. So in this case, eight goes into 16 twice and eight goes into eight once. Now, why is an empirical formula useful? Well, an empirical formula can be used to find the percent by mass of any atom in our compound. Now, let's look at this example again. How can we find the percent by mass of hydrogen in our molecule in our compound?