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Stoichiometry

Learn about stoichiometry, the measurement of elements and compounds in a reaction, through a series of questions and examples. Topics covered include mole ratios, mole-to-mole conversions, mass-to-mole conversions, and limiting reactants.

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Stoichiometry

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  1. Stoichiometry CDO High School

  2. Stoichiometry “stochio” = Greek for element “metry” = measurement Stoichiometry is about measuring the amounts of elements and compounds involved in a reaction. Consider the chemical equation: 4NH3 + 5O2 6H2O + 4NO There are several numbers involved. What do they all mean?

  3. Stoichiometry 4NH3 + 5O2 6H2O + 4NO With Stoichiometry we find out that 4 : 5 : 6 : 4 do more than just multiply atoms. 4 : 5 : 6 : 4 Are what we call a mole ratio.

  4. Stoichiometry 4NH3 + 5O2 6H2O + 4NO 4 : 5 : 6 : 4 Can mean either: 4 molecules of NH3 react with 5 molecules of O2 to produce 6 molecules of H2O and 4 molecules of NO OR 4 moles of NH3 react with 5 moles of O2 to produce 6 moles of H2O and 4 moles of NO

  5. Stoichiometry Equations

  6. Mole to Mole Coefficient Unknown Moles of Known Coefficient Known

  7. 6 mol H2O 5 mol O2 Stoichiometry Question (1) 4NH3 + 5O2 6H2O + 4NO • How many moles of H2O are produced if 2.00 moles of O2 are used? 2.00 mol O2 = 2.40 mol H2O Notice that a correctly balanced equation is essential to get the right answer

  8. 4 mol NO 6 mol H2O Stoichiometry Question (2) 4 NH3 + 5 O2 6 H2O + 4 NO How many moles of NO are produced in the reaction if 15 mol of H2O are also produced? 15 mol H2O = 10. mol NO

  9. Mole to Mass Coefficient Unknown Molar Mass Unknown Mole of Known Coefficient Known

  10. 59 g H2O = 6 mol H2O 18.02 g H2O 4 mol NH3 1 mol H2O Stoichiometry Question (3) 4 NH3 + 5 O2 6 H2O + 4NO • How many grams of H2O are produced if 2.2 mol of NH3 are combined with excess oxygen? 2.2 mol NH3

  11. Mass to Mole Coefficient Unknown Mass Known Molar Mass Known Coefficient Known

  12. 5 mole O2 1 mol H2O = 1.96 mol O2 6 mol H2O Stoichiometry Question (4) 4 NH3 + 5 O2  6 H2O + 4 NO • How many moles of O2 are required to produce 42.3g of H2O? 42.3 gH2O 18.02 g H2O

  13. Mass to Mass Conversions Molar Mass Unknown Mass Known Coefficient Unknown Coefficient Known Molar Mass Known

  14. 4 mol NO 30.01 g NO 5 mol O2 1 mol NO = 9.0 g NO Stoichiometry Question (5) 4 NH3 + 5 O2 6 H2O + 4 NO • How many grams of NO is produced if 12 g of O2 is combined with excess ammonia? 12 g O2 1 mol O2 32 g O2

  15. 3 mol Cu 63.55 g Cu 2 mol Al 1 mol Cu = 55.8 g Cu 3CuSO4 + 2Al  Al2(SO4)3 + 3Cu • What is the mass of copper formed when 15.8g of Aluminum is used? 15.8g Al 1 mol Al 26.98 g Al

  16. Have we learned it yet? 4 NH3 + 5 O2 6 H2O + 4 NO a) How many moles of H2O can be made using 1.6 mol NH3? b) what mass of NH3 is needed to make 0.75 mol NO? c) how many grams of NO can be made from 47 g of NH3?

  17. 6 mol H2O 30.01 g NO 4 mol NO 4 mol NH3 17.04 g NH3 13 g NH3 2.4 mol H2O ÷ 17.04g/molNH3 x x x x x = = 1 mol NH3 4 mol NO 4 mol NH3 4 mol NH3 1 mol NO = 83 g NO Answers 4 NH3 + 5 O2  6 H2O + 4 NO a) b) c) 1.6 mol NH3 0.75 mol NO 47 g NH3

  18. Reactions Involving aLIMITING REACTANT

  19. 2 NO(g) + O2 (g) 2 NO2(g) LIMITING REACTANTS Reactants Products Limiting reactant = ___________ Excess reactant = ____________

  20. Limiting Reactants: An Analogy • If you were going to make pb and j sandwiches, and you had a new loaf of bread and a large jar of pb and a large jar of jelly, how many sandwiches can you make? CDO IB Chemistry SL

  21. Example: Limiting Reactants • Mix 5.40 g of Al with 8.10 g of Cl2. What mass of AlCl3 can form? Al(s) + Cl2(g)  AlCl3 (s) CDO IB Chemistry SL

  22. How much of which reactant will remain when reaction is complete? • Cl2 was the limiting reactant. • Therefore, Al was present in excess. But how much? • First find how much Al was required. • Then find how much Al is in excess.

  23. Calculating Excess Al 2 Al + 3Cl2 2 AlCl3

  24. White Boards • In the reaction BaCO3+ 2HNO3 Ba(NO3)2 + CO2 + H2O, what mass of Ba(NO3)2 can be formed by combining 55g BaCO3 and 26g HNO3

  25. Percent Yield

  26. 2As2O3 + 3C 3CO2 + 4As • If 8.87g of As2O3 is used in the reaction and 5.33 g of As is produced, what is the percent yield?

  27. Gas Stoichiometry • Many chemical reactions involve gases as a reactant or a product • Gas Stoichiometry – the procedure for calculating the volume of gases as products or reactants • Gases also have a molar volume (L/mol) rather than concentration. • This is the conversion factor used to convert (liters of gas) to (moles of gas)

  28. Molar Volume STP = 22.4L/mol • Molar volume is the same for all gases at the same temperature and pressure (remember, all gases have the same physical properties) • At STP, molar volume = 22.4 L/mol • This can be used as a conversion factor just like molar mass! At STP, one mole of gas has a volume of 22.4 L, which is approximately the volume of 11 “empty” 2 L pop bottles.

  29. Volume of Gas to Volume of Gas Volume of Gas Known Volume of Gas Known Coefficient Unknown Coefficient Known Volume of Gas to Mass Molar Mass Unknown Volume of Gas Known Coefficient Unknown 1 mol 22.4 L Coefficient Known Mass to Volume of Gas Coefficient Unknown Mass Known 22.4 L Molar Mass Known Coefficient Known 1 mol

  30. Example #1 • If 300. 0g of propane burns, what volume of oxygen measured at STP is required for the reaction? C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) 300.0 g C3H8 1 mol C3H8 5 mol O2 22.4 L 44.11 g C3H8 1 mol C3H8 1 mol = 761.7 L O2

  31. Example #2 • Hydrogen gas is produced when sodium metal is added to water. What mass of sodium is necessary to produce 20.0L of hydrogen at STP? 2Na(s) + 2H2O (l)  2NaOH(aq) + H2(g) 22.99 g Na 1 mol 2 mol Na 20.0 L H2 22.4 L 1 mol Na 1 mol H2 = 41.1 g Na

  32. SOLUTIONS

  33. Solutions • Solute – what is being dissolved, usually present in the smallest amount • Solvent – what is doing the dissolving, usually present in the largest amount • Solution – solute and solvent combined • Aqueous solutions – solutions in which water is the solvent

  34. Concentration • Concentration – describes how much solute is dissolved in solvent • Saturated solution – when the solute can no longer dissolve in the given solvent • Concentration unit (mol/L) often referred to as Molarity (M) • [Concentration] = moles of solute volume of solution (L) CDO IB Chemistry SL

  35. Example: Concentration • A solution contains 4.10 g of NaCl in 1.00 L. What is the solutions concentration in mol/L.? CDO IB Chemistry SL

  36. Standard Solution CDO IB Chemistry SL

  37. Example: Making a Standard Solution • Calculate the mass of NaOH need to make 250 mL of a 0.200 mol/L solution. CDO IB Chemistry SL

  38. TITRATIONS

  39. Titrations • Laboratory Technique – which uses a standard solution to find the concentration of another solution.

  40. Titration Calculation n2M1V1 = n1M2V2 n= Coefficient of substance M = molarity of substance V = volume of substance

  41. Example: Titration What volume of 0.100 mol/L NaOH is required to titrate 25 mL of 0.300 mol/L solution of HCl to produce a neutral solution? CDO IB Chemistry SL

  42. H2SO4 + 2NaOH  Na2SO4 + 2H2O • What is the concentration of 25.85 mL of H2SO4 that is reacted with 21.50 mL of 0.300 mol/L solution of NaOH to produce a neutral solution?

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