Most every element comes in a few different isotopes, which have different numbers of neutrons and therefore different masses. But the periodic table only tells you the average mass. What's a chemistry student to do?!
Percent Composition of Compounds
In this short video we'll cover the only type of question that teachers seem to ask, which is how to find the mass percentage that each element in a compound is responsible for.
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...)