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CHAPTER 9 Chemical Quantities CHAPTER 9 TERMS Stoichiometry – the process of using a chemical equation to calculate the relative masses of reactants and products involved in a reaction. Mole ratios – conversion factors based on the balanced chemical equation (i.e., coefficients). Limiting reagent (reactant) – the reactant the is consumed first and limits the amounts of products that can be formed. Percent Yield – the actual yield of product compared to the theoretical yield, expressed as a percent. Theoretical Yield – the amount of substance that can be produced (mathematically speaking). Actual Yield – the amount of substance actually produced. STEPS FOR CALCULATING THE MASSES OF REACTANTS AND PRODUCTS IN CHEMICAL REACTIONS 1. 2. 3. 4. 5. 6. 7. Write the chemical equation. Do not forget to use phase labels. Balance the equation for the reaction. Write down the given information directly below the corresponding formula. Place a ? under the formula which the mass is unknown. Convert the masses of reactants or products to moles. Use the balanced equation to set up the appropriate mole ratio(s). Use the mole ratio(s) to calculate the number of moles of the desired reactant or product. STEPS FOR SOLVING STOICHIOMETRY PROBLEMS INVOLVING LIMITING REAGENTS. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Write the chemical equation. Do not forget to use phase labels. Balance the equation for the reaction. Write down the given information directly below the corresponding formula. Place a ? under the formula which the mass is unknown. Convert the masses of reactants to moles. Divide each the moles of the reactants by their respective coefficients. The smallest number is the limiting reactant. Use this number of find the required information. Use the balanced equation to set up the appropriate mole ratio(s). Use the mole ratio(s) to calculate the number of moles of the desired reactant or product. Convert from moles back to masses (grams, etc.) EXAMPLES 1. 2. 3. Calculate the mass of carbon dioxide produced when 96.1 g of propane (C3H8) reacts with sufficient oxygen. Calculate the mass of water formed by the complete reaction of 96.1 g of propane with oxygen. Hydrofluoric acid reacts with silicon dioxide to produce silicon tetrafluoride and water. a. b. How many grams of HF are needed to react with 5.68 g of silicon dioxide? Calculate the mass of water produced. EXAMPLES 4. 5. 6. Carbon monoxide reacts with hydrogen to produce methanol (CH3OH). How many grams of carbon dioxide and what mass of hydrogen are needed to form 6.0 kg of methanol? Lithium nitride is prepared by the reaction of lithium metal and nitrogen gas. Calculate the mass of lithium nitride formed from 56.0 g of nitrogen gas and 56.0 g of lithium. Titanium (IV) oxide can be prepared by reacting gaseous titanium (IV) chloride with oxygen gas. A second product of this reaction is chlorine gas. a. b. Suppose 6.71 x 103 g of titanium (IV) chloride is reacted with 2.45 x 103 g of oxygen. Calculate the maximum mass of titanium (IV) oxide that can form. If the percent yield of TiO2 is 75%, what mass is actually formed?