Sometimes a Problem Sounds Like You Are Being Given Too Much Information

Sometimes a problem sounds like you are being given too much information. This is really nothing more than a mass-mass problem with a limiting reactant, and you must decide which piece of data given to use to work the problem and which one to reject. There is no rule that says that every bit of both reactants will always be used up. Substances will combine with each other ONLY in stoichiometrically-correct amounts, and some of ONE of the reactant might be left over. ALL OF THE LIMITING REACTANT WILL BE USED UP, BUT THERE WILL BE SOME OF THE EXCESS REACTANT LEFT OVER.

There are several ways to approach working these problems. Instructions are below for two different ways of approaching limiting/excess reactant problems. You decide which method you understand and like best.

How to set up and solve limiting reactant problems Method 1

(1) Write a balanced equation for the reaction which is implied or given. (2) :If a problem gives quantities of BOTH reactants, it is a limiting reactant problem (3) Start with one of the reactants and work a mass-mass problem to find out how much product is produced. Now work the problem beginning with the other given reactant and find out how much of the same product as before is produced. The reactant which gives you the LEAST amount of product formed is the limiting reactant, and the LEAST amount of product is the true quantity of product which will form. This is based on the premise that a chain is only as strong as its weakest link; a reaction can only produce as much product as the LEAST amount of reactant used. (4) Be sure to mark clearly on your equation which is the limiting reactant and which is the excess reactant so you don’t have to try to remember which is which. (5) Now to find the amount of reactant which is left over, you begin with the correct amount of product which you just calculated and work backwards with a mass-mass problem to find the amount of the excess reactant which was USED to produce that amount of product. (6) Now subtract the amount of excess reactant USED (which you just calculated) from the initial given amount of reactant given, and you have the amount of reactant which is left over. You may have to manipulate your answer, depending on what units are asked for (moles, liters, grams of excess reactant). Second approach to solving limiting reactant problems: (1) Write a balanced equation for the reaction which is implied or given in the problem. (2) Find the number of MOLES of each reactant given and write it down so you can find it (3) select one of the reactants and do a mole-mole problem to find out if you have enough of the other reactant to react completely with it. This will tell you which of the reactants is the limiting reactant, and you should mark it clearly 4 (4) work the mass-mass problem beginning with the LIMITING 5 REACTANT (5) to find out how much of the excess reactant is left over, do one last problem in which you begin with the limiting reactant and calculate the mass of the excess reactant which will be USED in the reaction (6) subtract the amount of the excess reactant which was used in the reaction from the amount of the excess reactant you had to start with. It will be easier if you will do this step in MOLES and then convert it to what is asked for at the end of the problem. 6