 In this video we'll look at how to determine the empirical formulae of unknown molecules using their percent composition. But first, what is an empirical formula? You are of course by now very familiar with molecular formulae. They show the exact number and type of atoms that are bonded together to give a particular molecule. For instance, a water molecule contains two hydrogen atoms and an oxygen atom, so we write it as H2O. And peroxide has the crucial addition of another oxygen, so it's H2O2. And acetic acid, which when diluted with water gives vinegar, contains two carbon for hydrogen and two oxygen atoms in each molecule. So these are molecular formulae. The empirical formula of a compound shows the lowest whole number ratio of atoms in the molecule. For some molecules this is the same as its molecular formula. With water, for instance, we cannot simplify the ratio of hydrogen to oxygen any further, since we have only one oxygen anyway. So the empirical formula of water is still H2O. But in hydrogen peroxide the ratio of H2O is 2 to 2, which simplifies to 1 to 1. So the empirical formula is HO. Similarly, for acetic acid, all the numbers of atoms in the formula are divisible by 2. And the empirical formula is CH2O. Note that if you're dealing with an ionic compound the formula will already be an empirical formula, since it represents the lowest whole number ratio of ions in the ionic lattice. To give you an idea of the historical importance of this slightly odd way of looking at things, it was empirical formulae that were the first formulae that chemists were able to discover. In fact the word empirical means discovered by experiment. One of the methods early chemists used to analyze unknown substances was developed by Eustace von Liebig in 1831 specifically for carbon-based compounds. A sample would be thoroughly burnt in the presence of oxygen. Combustion of a carbon-based fuel produces carbon dioxide and water as products, so those product gases would flow first through a tube containing dry calcium chloride. This salt absorbs water, so from the increase in mass of this tube the scientist could work out how much water was produced and hence what mass of hydrogen had been in the original compound. The remaining gas would then flow through these bulbs with the lower ones containing potassium hydroxide solution. The potassium hydroxide absorbs carbon dioxide by reacting with it to produce potassium carbonate. So by the increase in mass of this part of the apparatus the scientist could work out how much carbon dioxide had been produced and hence what mass of carbon was in the original sample. Armed with these values and the mass of the original substance the chemist could work out the percentage by mass of carbon and hydrogen in the original sample. And from this information a simple empirical formula could be worked out. So let me show you how it's done. What we need to do is to turn the mass percentages from our experimental analysis into mole ratios. Since moles are just a way of skip counting atoms the mole ratio also gives us the whole atom ratio in the formula of the compound. In essence we're doing the opposite of what we do when we work out the mass percent of an element in a particular substance. There we took the formula of the substance and converted it into mass percentages for each element and here we're taking the mass percentages for each element and converting them back into a formula.