In this lesson, we studied the stoichiometry of chemical reactions, that is, the relationships between the amounts of the various chemical reactants and products that are involved in chemical reactions. 

We found that total mass was conserved in all chemical reactions.  This follows from the fact that in chemical reactions, atoms are neither created nor destroyed.  It is for this reason that we can balance chemical reactions by making sure that the total number of atoms of each element are the same among the reactants and products.

The balanced equation for a reaction therefore gives us all the information we need to determine the number of grams of any product formed or reactant consumed, given the number of grams of any other product formed or reactant consumed.

Since we don't necessarily always combine just the right amount of each reactant needed in a reaction, sometimes there are reactants - called excess reactants - left over.  It is the reactant that is used up - the limiting reactant - that determines when the reaction ends and the amount of each product that is formed.   We learned how to determined which reactant will be limiting, how much product will be formed, and how much of each excess reactant will be left over.

The percent, by mass, of each element in a compound is called its percent composition.  We found out how the percent composition of a compound can be determined from its formula and, in turn, how the percent composition (which we measured in the lab for magnesium oxide) can be used to determine the empirical formula of a compound.

Finally, we learned about two additional early laws of chemistry.  The Law of Constant Composition says that any sample of a given compound will have the same percent composition as any other sample of the same compound.  The Law of Simple Multiple Proportions is based on the observation that if two elements form more than one compound the ratio of their masses in one compound will form a series of simple, whole number multiples of the ratio of their masses in the other compounds.   Both of these laws imply that elements combine to form compounds in ways that are regular and predictable.