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# 104_lab9 - Dr Littles learning goals for this lab Calculate...

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Dr. Little’s learning goals for this lab: Calculate molar masses from formulas Find moles of liquid reactant using density and molar mass Find moles of a solid reagent using molar mass Find theoretical yield of a chemical reaction Determine limiting reactant from calculations (calculate how much product can be made from EACH reactant; the reactant that makes less product limits reaction and is limiting reactant) Calculate % yield % yield = actual yield (measured) x 100% theoretical yield (calculated) Understand why we don’t get 100% yield in most reactions

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A chemical reaction is a recipe. We need to be able to find out how much product we can make in the same way that we do with a baking recipe: If we have 6 crusts, how many pies can we make? (NOTE: also called theoretical yield) How many apples do we need? (NOTE: the word PER means divide by) With 8 crusts & 15 apples, how many pies can I make? Since 3 pies is less than 4 pies, the apples will run out before the crusts, and only 3 pies can be made. (NOTE: apples are the limiting reagent) apples pie apples pies 15 5 3 1 5 3 = × = × 2 crusts + 5 apples 1 pie pies apples pie apples pies crusts pie crusts 3 5 15 5 1 15 4 2 8 2 1 8 = = × = = × pies crusts pie crusts 3 2 6 2 1 6 = = ×
Stoichiometric Ratio The coefficients tell us how many moles of one reactant/product are related to how many moles of another. y and x are the coefficients in front of chemicals Y and X The ratio is called the “stoichiometric ratio” Y moles X moles x Y moles y X moles # # = ×

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Steps for Stoichiometry problems (Dimensional analysis method) Write correct formulas for all reactants and products, and balance the equation Identify the given substance (or substances ** ) and the desired substance Convert qty of given substance into moles (grams ÷ MM = moles) (if needed) Convert moles given into moles desired: (Use the coefficents in the balanced reaction for ? and ??) Convert the moles desired into alternate units (usually grams) if the problem asks (moles x MM = grams) ** if two or more givens, must do ALL STEPS for EACH, then pick smaller answer (limiting reactant) desired moles given moles desired moles given moles # ?? ? # = ×
Find moles of solid and liquid reactants Remember we have 3 mole equations: To find the moles of a liquid from the V of liquid, it is tempting to use the moles of solution formula BUT if we have a pure liquid, we need to use the moles of pure formula since there is no solution.

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