Working with Conversion Factors There are a number of conversion factors available to us in Chemistry. Avogadro's Number: converts between moles and molecules/ions/atoms Molar mass: converts between grams and moles Mole ratio: converts between moles of compounds in a chemical formula Stoichiometry: converts between moles of compounds in a chemical equation This tutorial will focus on the last conversion factor, Stoichiometry. If you need help with the other conversion factors, see the Grams to Moles Tutorial This tutorial is arranged as follows. You will be given a brief explanation of what stoichiometry means along with an example problem. After the example problem, you will be asked to work a problem on your own. If you work this problem correctly, you will be given an additional problem of higher difficulty. You can continue working these problems until you feel you understand the material. (Click on the "End Tutorial" button at the bottom of the page). If you do not get the correct answer, you will be given a new problem that is a bit easier than the previous problem. If you are really struggling to get the correct answer, the tutorial will send you back to this page to review the material again. Hopefully, by working problems of various difficulty, you will see a variety of ways that Stoichiometry problems are used in Chemistry. Ready? OK, let's begin. Stoichiometry is the study of the amounts of materials consumed and produced in a chemical reaction. To convert from moles of one compound to moles of any other compound, multiply by the mole ratio given by the stoichiometric coefficients in the balanced equation. Whew! There are lots of terms in the statement above. Let's look at what they all mean. First, stoichiometry is a conversion factor that relates chemical compounds in a balanced equation. That is to work stoichiometry problems, you must have some equation and that equation must be balanced. For example: NaOH + HCl ---> NaCl + H2O Given this balanced equation, we can now relate the number of moles of any one compound (in the equation) to any other compound (in the equation). The stoichiometric coefficients are the numbers in front of each compound in the balanced equation. In our example above, the stoichiometric coefficients for NaOH, HCl, NaCl, and H2O are all "1". To use stoichiometry, all we do is multiply by the mole ratio of the compounds of interest. And the mole ratio is determined from the stoichiometric coefficients. For example: The mole ratio between NaOH and HCl from our example equation is 1 mol NaOH/ 1 mol HCl Why? Because the stoichiometric coefficients of each from the balanced equation are "1". Now let's use this information to work an example problem. The fertilizer ammonium sulfate is prepared by the reaction between ammonia and sulfuric acid: 2 NH3 (g) + H2SO4 (aq) --> (NH4)2SO4 (aq) How many moles of NH3 are needed to make 32 moles of ammonium sulfate? Remember, this is a conversion factor problem, so we will set up the problem like we do for all problems of this type. (If you need help setting up conversion factor problems, you might want to look over the Grams to Moles Tutorial.) First, we need to determine what we are given and what we are looking to find. Then we need to figure out the conversion factor(s) needed to make the necessary conversions. Given: 32 moles (NH4)2SO4 Solving for: moles of NH3 Conversion Factor(s): stoichiometry of ammonia to ammonium sulfate: 2 mol NH3/ 1 mol (NH4)2SO4 Setup and Solve as: (Information Given) * (Conversion Factors) = (Information Solving For) or 32 moles (NH4)2SO4 * (2 mol NH3/ 1 mol (NH4)2SO4) = 64 moles NH3 Now it's your turn to work a few examples. You will need some scratch paper, a calculator, and a periodic table handy to do these examples. Important Notice: The following problems require that your browser is capable of handling javascript. If you do not see any information on the following pages, you will need to use a browser that can handle javascript. (Netscape browsers ver. 2.0 or higher and Internet Explorer browsers ver. 3.0 or higher will work.) Remember, you can exit this tutorial at any time by clicking on the "End Tutorial" link at the bottom of the page. When you are ready, click on the "Continue Tutorial" link below.