• Limiting Reagent: – The reactant that limits the amount of product that can be formed in a chemical reaction – Always based on the # of molecules or moles present
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• Two factors that are important in determining the limiting reagent – the number of moles present of each of the reactants – the mole ratio of the reactants
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• Calculating the limiting reagent when given mass or grams 1. Convert both quantities to moles (use molar mass) 2. Determine limiting reagent using quantities of MOLES of reactants (use mole ratio) 3. Use the limiting reagent to do further calculations
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• 2Cu + S → Cu2S – What is the limiting reagent 80.0 g Cu reacts with 25.0 g S? – Step 1: convert both to moles:
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Step 2: Based on quantity of MOLES, determine the limiting reagent:
2Cu + S → Cu2S
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• Copper is the limiting reagent: – Determine the maximum number of grams of Cu2S that can be produced from the reaction: 2Cu + S → Cu2S 1.26 mol Cu is the limiting reagent
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• Calculating percent yield is similar to calculating the percent of correctly answered questions on a test. – If a student answered 18 out of 20 questions correctly, what percentage correct did they answer?
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• Calculating Percent Yield – Sometimes things “go wrong” in chemical reactions – The actual yield is usually less than the theoretical yield – A percent yield should generally not be larger than 100% – Percent yield refers to the PRODUCTS of a reaction Slide 9 of 16
• Calculating percent yield:
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• What is the percent yield if you combined .164 mol Mg with an excess of HCl and .160 g H2 was produced? The theoretical yield is 0.164 g H2
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• CaCO3 → CaO + CO2 – What is the % yield of this reaction if 24.8 g of CaCO3 is heated to produce 13.1 g CaO – Calculate the theoretical yield of g CaO: mass to mass conversion
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• Then calculate the percent yield: – 13.1 g CaO was produced. – Theoretical yield = 13.9 g CaO
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When 50.0 g of SiO2 reacts with an excess of carbon, 27.9 g of SiC is produced. What is the percent yield?
SiO2 + 3C → SiC + CO2 1. Calculate theoretical yield of g SiC 2. Calculate percent yield
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• Q 48 and 52 pg 379‐380 • WS: Practice Problems 12.3 due Thurs. • Complete and check all lab calculations with your group.