Activity 151-9 Introduction to toichiometry Directions: This Guided Learning Activity (GLA) focuses on completing stoichiometric calculations. Part A discusses the relationships that allow us to convert between chemical quantities, Part B introduces basic stoichiometric calculations, and Part C discusses limiting reactant problems. The worksheet is accompanied by instructional videos. ee http://www.canyons.edu/departments/chem/gla/ for additional materials. Part A The Basics toichiometry utilizes quantitative relationships to determine the mass, concentration, volume or amount of a substance that is produced or consumed during a given chemical process. It can be applied to a wide range of chemical quantities. For each connection below, give the conversion factor that can be used. An example conversion for Volume of A to Mass of A is shown. density of A = ( g ml ) Coefficients in Reaction Avogadro's # Avogadro's # Activity 151 9 Page 1 of 4
Part B Mass-to-Mass Conversions Using the conversion factors from part A, the amount of a product in a chemical reaction can be calculated. Example #1a. If 2.54 g of copper reacts with sulfur, how much copper (I) sulfide will be produced? 2Cu (s) + (s) Cu 2 (s) Mass Cu Moles Cu Moles Cu 2 Mass Cu 2 Cu Reaction Coefficients Cu 2 2.54 g Cu 1 mol Cu ( ) ( 1 63.55 g Cu ) (1 mol Cu 2 159.16 g Cu 2 ) ( 2 mol Cu 1 mol Cu2 ) = 3.18 g Cu 2 produced Example #1b. In the above problem, how many grams of sulfur were consumed? Mass Cu Moles Cu Moles Mass Cu Reaction Coefficients 2.54 g Cu ( ) ( ) ( ) ( ) = g consumed 1 Example #1c. What volume of copper is needed to produce 58.7 g of Cu 2? (d cu = 8.96 g/cm 3 ) ( 58.7 g Cu 2 ) ( ) ( ) ( ) ( ) = cm 3 Cu needed 1 Activity 151 9 Page 2 of 4
Part C Limiting Reactants When two or more reactants are combined, usually one of the reactants will be limiting, meaning it will run out first. When this happens, no more product will form. To determine the amount of product that forms, we must first determine which of several reactions will be depleted first, and use that quantity to calculate the amount of product that could form. Example #2a. How many grams of ammonia (NH 3) can be produced when 50.0 g of hydrogen gas and 25.0 g of nitrogen gas are mixed? 3H 2 (g) + N 2 (g) 2NH 3 (g) tep 1. Find how much NH 3 could be produced if all of the H 2 is used: tep 2. Find how much NH 3 could be produced if all of the N 2 is used: tep 3. Compare the calculated values. The reactant that produces the least amount of product is the limiting reactant. The limiting reactant in this reaction is. The calculated amount of product from the limiting reactant is the theoretical yield. It represents the maximum amount of product that could be produced by the reaction. The theoretical yield for NH 3 is g NH 3. Example #2b. If only 23.4 g of NH 3 was recovered from the reaction, what was the percent yield? The percent yield for a reaction quantifies the percent of the theoretical yield was achieved. Percent Yield = Actual Yield x 100% Theoretical Yield % Yield = x100% = % Activity 151 9 Page 3 of 4
Part D Extra Practice 1. 9.60 g of sodium metal is tossed into a stream. How many grams of sodium hydroxide are produced? 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2 (g) 2. Potassium chlorate is heated to decompose into potassium chloride and oxygen gas. How many grams of potassium chloride will remain if 391 g of potassium chlorate are heated and the reaction is allowed to go to completion? 2KClO 3 (s) 2KCl (s) + 3O 2 (g) 3. Urea is produced from a reaction between ammonia and carbon dioxide. A worker mixes 68.2 kg of ammonia and 105 kg of carbon dioxide, and produces 87.5 kg of urea. What is the percent yield for this reaction? 2NH 3 (aq) + CO 2 (aq) (NH 2 ) 2 CO (aq) + H 2 O(l) 4. Aspirin (C 9H 8O 4) can be synthesized by the following reaction: C 4 H 6 O 3 (aq) + C 7 H 6 O 3 (aq) C 9 H 8 O 4 (aq) + C 2 H 4 O 2 (aq) Aspirin A typical percent yield for this reaction is 65%. If salicylic acid (C 7H 6O 3) is the limiting reactant, how much salicylic acid is needed to synthesize 250 mg of aspirin? Challenge Problems. To complete these stoichiometry problems, first write and balance the relevant chemical reaction. For extra guidance, refer to the Writing Chemical Reactions GLA. 5. 22.3 g of calcium hydroxide is placed in a beaker containing 1.00 mol of hydrochloric acid. How much water is produced? Does the final solution contain excess Ca(OH) 2 or HCl? 6. You want to recover the silver ions from an aqueous solution. The solution contains 0.1 mole of AgNO 3, and you plan to add enough MgCl 2 to the solution to convert all of the AgNO 3 to AgCl. How many grams of MgCl 2 should you add? Activity 151 9 Page 4 of 4
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