Stoichiometry is the study of the quantities of reactants and products involved in a chemical reaction. In addition to identifying the reactants and products in a chemical reaction, a balanced equation gives useful information that is helpful in calculations. Consider the equation for the reaction between ammonia and oxygen to produce nitrogen (II) oxide: 4 NH3(g)+5 O2(g) −→4 NO (g)+6 H2O (g) The following information can be obtained: 1. Molecules of reactant and products: 4 molecules of NH3 react with 5 molecules of O2 to form 4 molecules of NO and 6 molecules of H2O. 2. Moles of reactants and products: 4 mol of NH3 react with 5 mol of O2 to produce 4 mol of NO and 6 mol of H2O. 3. Mass of reactants and products: 68 g of NH3 (4 mol) react with 160 g of O2 (5 mol) to produce 120 g of NO (4 mol) and 108 g of H2O (6 mol). 4. Volumes of gases: 4 volumes of NH3 react with 5 volumes of O2 to produce 4 volumes of NO and 6 volumes of H2O at the same temperature and pressure (by Avogadro’s law, which will be discussed in detail in section 11.5 of chapter 11). There are several types of stoichiometric problems. The common types include: 1. Mole – Mole 2. Mass – Mass 3. Mass – Mole (or Mole – Mass) 4. Mole – Volume (or Volume to Mole) 5. Mass – Volume (or Volume to Mass) 6. Volume – Volume The following general steps can be used to solve many stoichiometric problems: 1. Write the balanced chemical equation for the reaction. 2. Organize your data; determine which quantities you know and which ones you need to find. 3. Write the mole relationship between the given substance (a reactant or a product) and the required substance (a reactant or a product). 4. Calculate molar masses and convert masses, molecules, or volumes of the known substance to moles. 5. Use stoichiometric coefficients or conversion factors (mole ratios) from the equation to determine the moles of the unknown substance. 6. Convert moles of the unknown substance to the desired mass, molecules, or volume.
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