This video explains what the empirical formula of a compound is, and how to find it based on the molecular formula. An important first step!
Calculating Molecular Formula From Empirical Formula
You won't usually see this type of problem on its own -- it's usually just a step in a longer problem -- but since those longer problems always tend to be super tricky, it's a good idea to master this step first.
Finding Molecular Formula Using Percent Composition
These are the big nasty word problems we've been building towards. They look super-hard when you first see them, but by breaking them down into the step-by-step process in this video, most students end up looking forward to these on the test as "easy" points. Compared to Stoichiometry, anyways!
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)