 In this video, you're going to learn the difference between empirical formula and molecular formula, how to find the empirical formula from percentage composition by mass, and how to find molecular formula from the empirical formula. Consider the compound hydrazine. Its molecular formula is N2H4, but its empirical formula is NH2. The molecular formula of a compound tells us two things. One, the types of atom in the molecule, in this case nitrogen and hydrogen, and two, how many types of each atom? In this case, there are two nitrogen atoms and four hydrogen atoms. The empirical formula of a compound also tells us two things. One, the types of atoms in the molecule, in this case nitrogen and hydrogen, and two, the simplest ratio of each type of atom. The ratio of nitrogen to hydrogen in hydrazine is 2 to 4, making the simplest ratio 1 to 2. The best way to find the empirical formula from percentage composition by mass is to go over a question. Upon analysis, the percentage composition by mass of a compound was found to be 87.5% nitrogen and 12.5% hydrogen. Now calculate the empirical formula. Firstly, we have a column for each element, N for nitrogen and H for hydrogen. Then, under each element we put the percentage composition by mass. So that's 87.5% for nitrogen and 12.5% for hydrogen. We then divide these percentages by the relative atomic mass of each element. So that's 14 for nitrogen and 1 for hydrogen. Doing this calculation gives us 6.25 for nitrogen and 12.5 for hydrogen. We then divide these numbers by the smallest of the two, which is 6.25, and this gives us the ratio of 1 to 2. 1 and 2 is the smallest possible ratio, so the empirical formula is NH2. But how do we find the molecular formula? The molecular mass for this compound is 32. The molecular formula is the empirical formula times N, where N is the number by which we have to multiply the empirical formula in order to get the molecular formula. I have coined the phrase empirical mass to describe the mass of the empirical formula. So in this case we have nitrogen, which is 14, plus hydrogen, which is 2, giving us 16. Now, just as the molecular formula is the empirical formula times N, the molecular mass is the empirical mass times N. So molecular mass of 32 is N times the empirical mass, 16. Therefore N is 32 divided by 16, or 2. So now molecular formula is the empirical formula times N, so that's NH2 times N, or NH2 times 2, which gives us N2H4. So to recap, the molecular formula tells us the types of atoms in a molecule and their exact number, and the empirical formula tells us the types of atoms in the molecule and their simplest ratio.