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MASS RELATIONSHIPS IN CHEMICAL EQUATIONS

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

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

  2. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : CO2(g) + LiOH(s) Li2CO3(aq) + H2O(l) Step 1: Write a balanced chemical equation

  3. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : Step 2: Write what has been given, write what is unknown/needed.

  4. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : Step 3: Find the number of moles of each given/known compound.

  5. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : Step 4: Set up a mole ratio in order to solve for the number of moles of unknown (LiOH) required.

  6. EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : EXAMPLE 1: Determine the mass of lithium hydroxide required to react with 8.8 x 102 g of CO2 : Step 5: Find the mass of lithium hydroxide required by using the moles and molar mass.

  7. EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? STEP 1: Write out the balanced chemical equation & state what is given/needed.

  8. EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? STEP 2: Find the number of moles of the known (N2).

  9. EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? STEP 3: Use the balanced equation to find the mole ratio of the known/given substance to moles of unknown/needed substance (NaN3).

  10. EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? EXAMPLE 2: An airbag is inflated with nitrogen produced from the decomposition of sodium azide, NaN3. The mass of N2 in a fully inflated airbag is 87.5g. What mass of NaN3 is required to produce this mass of N2? STEP 4: Find the mass of sodium azide required by using the moles and molar mass.

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

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

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

  14. Example # 1 HBrO3(aq) + 5HBr(aq) → 3H2O(l) + 3Br2(aq) 20.0g of HBrO3 is reacted with excess HBr. • What is the theoretical yield of Br2 for this reaction? • If 47.3g of Br2 is produced, what is the percentage yield for Br2? Answers: • 74.4 g • 63.6%

  15. Example # 2 Using Hg(NO3)2(aq) + Na2S(aq) →HgS(s) + 2NaNO3 (aq), If 3.45x1023 molecules of Hg(NO3)2 are reacted with excess Na2S, what mass of HgS can be expected if this process occurs with 97.0% yield? Answer: 129g

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

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

  18. HOMEWORK:

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