

Using Balanced EquationsNow that you know how to balance equations, you need to know what to do with them, how you can use them. The way that you will use them is to come up with the weight relationships that exist between various chemicals involved in chemical reactions. In the following table (also shown in example 4 in your workbook), the first line gives the word equation "magnesium plus oxygen gives magnesium oxide." The second line gives the balanced equation "2 Mg + O_{2} 2 MgO." The coefficients in front of the formulasthe "2" in front of magnesium, the "1" that's not shown in front of oxygen, and the "2" that's shown in front of magnesium oxidecan be interpreted in terms of how many atoms and molecules react or in terms of how many moles of these chemicals will react with one another. Because of the way a mole is defined, if two magnesium atoms react with one oxygen molecule, then two moles of magnesium atoms will react with one mole of oxygen molecules. When two moles of Mg react with one mole of O_{2}, two moles of MgO will be formed. Once you have the balanced equation, the coefficients immediately give you the mole relationship. Two moles of magnesium reacts with one mole of oxygen to give two moles of magnesium oxide.
Since you already know how to figure the formula weights of compounds, you can use that along with the number of moles to find out the weight relationships that are involved in the chemical reaction. One mole of magnesium weighs 24.3 grams; therefore, two moles of magnesium weighs 48.6 grams. Oxygen has an atomic weight of 16.0, so the formula weight for O_{2} is 32.0 grams. Going over to the right side of the formula weight of magnesium oxide is the sum of the atomic weight of magnesium and the atomic weight of oxygen, which is 40.3. That is the weight in grams of one mole of magnesium oxide. Thus the weight of two moles of magnesium oxide is 80.6 grams. Note that we can determine the weight relationship in a chemical reaction just from knowing what the chemicals are. If we know the formulas of all the chemicals involved, we can write a balanced equation. From the balanced equation, we can figure out the mole relationship. From the number of moles of each chemical that are involved in the reaction and their formula weights, we can figure out the weight of each one of these. So from the balanced equation, you can get the weight relationships. These are the same kind of weight relationship that you were using a few lessons ago when you were working with calculations that involved the weight relationships in chemical reactions. Now, you can do that starting with a balanced equation or actually even starting with an unbalanced equation because you can balance the equation. You don't have to carry out the reaction and weigh the chemicals or be told what the weight relationship is. You can figure that out doing the very kind of thing that we just went through in this example. The next five examples show how to set up calculations to answer a variety of questions using information from a balanced equation. All of these examples use the equation for the formation of magnesium oxide. The questions are listed in example 5 in your workbook. If you wish, try your hand at answering these questions and then check your answers by looking at the following examples. Or you can begin by following through the worked out examples to see how they are done. By the way, the mole and weight relationships in chemical reactions based on balanced equations are called stoichiometry. After you have worked through these examples, try your hand with the practice problems (also given in Exercise 6 in your workbook) and check your answers on the "Answers" page.
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