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MASS RELATIONSHIPS IN CHEMICAL EQUATIONS STOICHIOMETRY - the study of the mass and amount relationships between reactants and products in a chemical reaction.

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EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 10 2 g of CO 2 : CO 2(g) + LiOH (s) Li 2 CO 3(aq) + H 2 O (l) Step 1: Write a balanced chemical equation

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EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 10 2 g of CO 2 : Step 2: Write what has been given, write what is unknown/needed. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 10 2 g of CO 2 :

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Step 3: Find the number of moles of each given/known compound. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 10 2 g of CO 2 :

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Step 4: Set up a mole ratio in order to solve for the number of moles of unknown (LiOH) required. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 10 2 g of CO 2 :

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Step 5: Find the mass of lithium hydroxide required by using the moles and molar mass. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 10 2 g of CO 2 :

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EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN 3. The mass of N 2 in a fully inflated airbag is 87.5g. What mass of NaN 3 is required to produce this mass of N 2 ? STEP 1: Write out the balanced chemical equation & state what is given/needed. EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN 3. The mass of N 2 in a fully inflated airbag is 87.5g. What mass of NaN 3 is required to produce this mass of N 2 ?

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STEP 2: Find the number of moles of the known (N 2 ). EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN 3. The mass of N 2 in a fully inflated airbag is 87.5g. What mass of NaN 3 is required to produce this mass of N 2 ?

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STEP 3: Use the balanced equation to find the mole ratio of the known/given substance to moles of unknown/needed substance (NaN 3 ).

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EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN 3. The mass of N 2 in a fully inflated airbag is 87.5g. What mass of NaN 3 is required to produce this mass of N 2 ? STEP 4: Find the mass of sodium azide required by using the moles and molar mass. EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN 3. The mass of N 2 in a fully inflated airbag is 87.5g. What mass of NaN 3 is required to produce this mass of N 2 ?

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No way!! Do you think that in the chemistry lab that we always get the amount of product that we calculate?

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actual amount of a product expressed as a % of the theoretical amount of that substance % Yield

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Terms: Theoretical yield: The amount of product predicted by stoichiometry. Actual yield: The amount of product produced through experimenting. Percentage yield:

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Example # 1 HBrO 3 (aq) + 5HBr(aq) → 3H 2 O(l) + 3Br 2 (aq) 20.0g of HBrO 3 is reacted with excess HBr. a) What is the theoretical yield of Br 2 for this reaction? b) If 47.3g of Br 2 is produced, what is the percentage yield for Br 2 ? Answers: a) 74.4 g b) 63.6%

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Example # 2 Using Hg(NO 3 ) 2 (aq) + Na 2 S(aq) → HgS(s) + 2NaNO 3 (aq), If 3.45x10 23 molecules of Hg(NO 3 ) 2 are reacted with excess Na 2 S, what mass of HgS can be expected if this process occurs with 97.0% yield? Answer: 129g

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Application of Percentage Yield o Very important for chemical engineers who must produce large quantities of product. o An increase in percentage yield can result in increased profits. Percentage Purity o Typically applied to pharmaceuticals and the mining industry. Using Percent Yield

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What do your numbers mean? A percentage yield less than 100% means o Other side reactions are happening at the same time that are not expected. o Decomposition of reactants or unreacted reactants = Realistic A percentage yield greater than 100%: o You have created matter (impossible) = Experimental error

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HOMEWORK:

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