Chapter 13 – Stoichiometry

Stoichiometry (STOY-key-OM-etry) problems are based on quantitative relationships between the different substances involved in a chemical reaction. 13.1 Mole Ratio The coefficients in a balanced equation given the moles of each substance in that equation. For the combination reaction of hydrogen gas and nitrogen gas to produce ammonia, the coefficients give us valuable information about the reaction: N2(g) + 3 H2(g)  2 NH3(g) For every 1 molecule of nitrogen that reacts, it needs three molecules of hydrogen to react with it. Together, they produce 2 molecules of ammonia, NH3. + +  We can also say for every 1 mole of N2 that reacts, 3 moles of H2 reacts with it to produce 2 moles of NH3.  These are mole-to-mole relationships/ratios. o Given a balanced equations; any two compounds can be compared using mole-to-mole relationships or mole ratios. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) The mole ratios would be: 3 mol CO 1 mol C H 3 mol CO ( 5 mol O 2) and ( 4 mol H3 O8 ) and (4 mol H O2 ) and 2 2 5 mol O 5 mol O2 (4 mol H 2O) and ( 1 mol C 3 H8 2 2 3 mol CO2 ) and ( 1 mol C 3 H8 ), etc. These mole ratios are used to solve problems such as how many moles of carbon dioxide, CO2, would be produced from 6.25 moles of oxygen gas? 3 mol CO2 Solution: 6.25 moles O2 ( 5 mol O2 ) = 3.75 moles CO2 YouTube Video: Solving Stoichiometry Problems by weiner7000 STOP at 7:25 until you have read through the next three sections. page 1 13.2 Mass-Mass Stoichiometry Grams of Given Molar Mole-Mole Moles of Given Mass Molar Moles of Unknown Ratio Mass Grams of Unknown Steps: 1) Grams of given  moles of given (Use the MM of given as your conversion factor.) 2) Moles of given  moles of unknown (Use mole ratios from balanced equation.) 3) Moles unknown  grams unknown (Use the MM of unknown as your conversion factor.)  Important to include units & formulas for all substances- units cancel except wanted units. Example: Calculate the mass of H2 required to react with 8.75 g of O2 according to the following balanced equations: O2(g) + 2 H2(g)  2 H2O(g) Answer: 8.75 g O2 ( 1 mol O2 32.00 g O2 2 mol H 2.02 g H ) (1 mol O2) ( 1 mol H 2) = 1.10 g H2 2 2 (In your calculator: 8.75 ÷ 32.00 × 2 × 2.02 =) 13.3 Mass-Volume Stoichiometry Grams of Given Molar Moles of Given Mass Mole-Mole Ratio Moles of Unknown Molar Volume gas @ STP Liters of Unknown OR Liters of Given Molar Moles of Given Volume gas @ STP Mole-Mole Ratio Moles of Unknown Molar Mass Mass of Unknown Recall: Avogadro’s Molar Volume is 22.4 L/mol for a gas only at STP Steps: 1) If given grams, use MM as your conversion factor to get to moles of the given -If given volume, use molar volume to get to moles of the given 2) Use mol ratios to convert from moles of given to moles of unknown 3) If asked to find grams, use MM as your conversion factor to get to grams of the unknown -If asked to find volume, use molar volume to get to liters of the unknown Example: How many liters of oxygen gas are needed to react with 0.234 grams of SO2 gas at STP? 2 SO2(g) + O2(g)  3(g) Answer: 0.234 g SO2 ( 1 mol SO2 64.07 g SO2 1 mol O 22.4 L O ) (2 mol SO2 ) ( 1 mol O 2) = 0.0409 L O2 2 2 (In your calculator: 0.234 ÷ 64.07 ÷ 2 × 22.4 =) page 2 13.4 Volume-Volume Stoichiometry Liters of Given Molar Volume gas @ STP Mole-Mole Moles of Given Moles of Unknown Ratio Molar Volume gas @ STP Liters of Unknown Fact: If you start with liters of the given and are asked to find liters of the unknown, as long as the gases are at the same temperature and pressure the molar volumes will cancel out with each other so you are basically just using the mole ratio to solve this type of problem. Example: How many liters of oxygen gas are needed to produce 36.5 liters of SO3 gas at STP? 2 SO2(g) + O2(g)  3(g) Answer: 36.5 L SO3 ( 1 mol SO3 22.4 L SO3 1 mol O 22.4 L O ) (2 mol SO2 ) ( 1 mol O 2) = 18.3 L O2 3 2 (notice molar volume cancels out with itself on this problem) 36.5 L SO3 ( 1 mol O2 2 mol SO3 ) = 18.3 L O2 Putting them all together you get this chart: YouTube Video: Solving Stoichiometry Problems by weiner7000 CONTIUNUE from 7.25 for more examples  page 3 CHAPTER 13 PRACTICE PROBLEMS Example 1: N2 (g) + 3 H2 (g)  2 NH3 (g) A. How many moles of N2 are needed to completely react with 6.75 moles of H2. B. How many moles of NH3 form when 3.25 moles of N2 react? C. How many moles of H2 are required to produce 4.50 moles of NH3? Example 2: Consider the following reaction to produce iron, Fe (s): Fe2O3 (s) + 3 CO (g)  2 Fe (s) + 3 CO2 (g) A. Calculate the mass of CO needed to react completely with 50.0 g of Fe2O3 . B. Calculate the mass of iron produced when 125 g of CO reacts completely. C. Calculate the mass of CO2 produced when 75.0 g of iron is produced. Example 3: Calculate the volume (in liters) of oxygen gas required to react with 50.0 g of aluminum at STP. 4 Al (s) + 3 O2 2 Al2O3 (s) Example 4: An automobile airbag inflates when N2 gas results from the explosive decomposition of sodium azide (NaN3), 2 NaN3 (s) 2 Na (s) + 3 N2 (g) Calculate the mass of NaN3 required to produce 50.0 L of N2 gas at STP. Answers to Practice Problems  1 mol N  2  Example 1 A 6.75 moles H2    2.25 mol N2 3 mol H 2    2 mol NH  3   6.50 mol NH3 B 3.25 moles N2  1 mol N 2    3 mol H  2  C 4.50 moles NH3    6.75 mol H2 2 mol NH 3   page 4  1 mole Fe2O3  3 mole CO  28.01 g CO     = 26.3 g CO  159.70 g Fe2O3  1 mole Fe2O3  1 mole CO  Example 2 A 50.0 gFe2O3   1 mole CO  2 mole Fe  55.85 g Fe     = 166 g Fe  28.01 g CO  3 mole CO  1 mole Fe  B 125 g CO  1 mole Fe  3 mole CO2  44.01g CO2   = 88.7 g CO2    55.85 g Fe  2 mole Fe  1 mole CO2  C 75.0 g Fe  1 mole Al  3 mole O2  22.4L O2   = 31.1 L O2    26.98 g Al  4 mole Al  1 mole O2  Example 3 50.0 g Al  mol N2  2 mol NaN3  65.02 g NaN3    96.8 g NaN3    22.4L N2  3 mol N2  1 mol NaN3  Example 4 50.0L N2  page 5