[Free Registration/Login may be required to access all resource tools.] In this online tutorial students will learn to calculate the percent composition of a compound either from mass data or from the chemical formula. They will use percent composition to calculate the mass of an element in a certain sample of a compound and calculate the percentage of a hydrate's mass that is due to water. Students will determine the empirical formula of a compound from percent composition data and the molecular formula of a compound from the empirical formula and the molar mass.