Stoichiometry allows us to use ratio and proportion to determine the amount of product in a chemical reaction when the amount of reatant is known, OR the amount of reactant needed to react with another given amount of reactant, OR the amount of reactant needed to produce a certain amount of product. These concepts are also used for calculations of percent yield, limiting reactants, excess reactants, energy absorbed or emitted, concentrations, equilibriums, etc. Thus it becomes a basic skill to be applied in solving many other types of problems. In order to help you to become proficient in this skill, I have provided three sample problems below, after which you should try to solve the additional problems. Do not check your answers until you have tried all five of the problems.
Magnesium reacts readily with hydrochloric acid to produce magnesium chloride and hydrogen gas.
This will calculate the number of moles of the unknown amount. In other words, you are given a certain mass of reactant, and you are asked to find how much product to expect from the given reactant amount. Since we have an excess of HCl, we do not need to be concerned with it since we know we will have all we need for the amount of Mg given. Also, since no information is given about the magnesium chloride and we do not need to find out anything about it, then we do not need it in soving the problem except for the fact that we need to consider it in the first step when writing our balanced equation. This is a single replacement reaction in which the iron will be replaced by the carbon.