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Stoichiometry (S)

Stoichiometry (S). Composition S: Mass relationships in compounds Reaction S: Mass relationships between reactants and products To find amounts of products and/or reactants, you must convert both to moles!. Mole Ratio.

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Stoichiometry (S)

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  1. Stoichiometry (S) • Composition S: Mass relationships in compounds • Reaction S: Mass relationships between reactants and products • To find amounts of products and/or reactants, you must convert both to moles! Mullis

  2. Mole Ratio A mole ratio is a conversion factor that relates the amounts of moles of any 2 substances involved in a chemical reaction. 2Al2O3(l) 4Al(s) + 3O2(g) Possible mole ratios: 2 mol Al2O3 or 4 mol Al 4 mol Al 2 mol Al2O3 2 mol Al2O3 or 3 mol O2 3 mol O2 2 mol Al2O3 4 mol Al or 3 mol O2 3 mol O2 4 mol Al Mullis

  3. Ideal Condition • In chemical reactions, all reactants are converted to products under ideal conditions. • Solution plan if you have a given and unknown quantity in moles: Given mol x Mole ratio unknown mol = Unknown mol given mol Mullis

  4. Given mol x Mole ratio unknown mol = Unknown mol given mol 2Al2O3(l) 4Al(s) + 3O2(g) If start with 3 moles aluminum oxide, how much Al will be obtained? 3 mol Al2O34 mol Al = 6 mol Al 2 mol Al2O3 6 mol Al 26.98 g Al = 161.88 g Al 1 mol Al Mullis

  5. Conversion Reminders Mullis

  6. X moles H2O 1 mole O2 2 moles H2O • moles A  moles B 2 steps X grams H2 X moles H2O 1 mole H2 1 mole O2 32 g O2 1 mole O2 • grams A  moles B 3 steps • moles A  grams B 3 steps X grams H2 2 moles H2O 2.02 g H2 1 mole H2 2 moles H2 1 mole O2 1 mole O2 32 g O2 • grams A  grams B 4 steps 2.02 g H2 2 mole H2 1 mole O2 4 Problem Types Mullis

  7. Practice Problems 1. When sodium azide (NaN3) is activated in an automobile airbag, nitrogen gas and sodium are produced. If 0.500 mol NaN3 react, what mass in grams of nitrogen would result? 2. Carborundum, SiC, is a hard substance made by combining silicon dioxide with coke (C). The products are SiC and CO. What is the mass of SiC in grams from the complete reaction of 2.00 mol carbon? Mullis

  8. Practice Problems: Stoichiometry Set 4 • How many grams of sodium should be added to 50 g fluorine to make sodium fluoride? • Mg(OH)2 + 2HCl 2H2O + MgCl2 This equation represents the neutralization of stomach acid by milk of magnesia. • What is the mass in grams of MgCl2 which will be produced if 3.00 g of Mg(OH)2 reacts? • What mass in grams of HCl is required to completely react with 3.00 g of Mg(OH)2? Mullis

  9. % Yield • % Yield = Actual – Expected x 100% Expected Which is the same as: • % Yield = Observed–Theoretical x 100% Theoretical To find Expected, or Theoretical, yield: Do a gram-to-gram problem. Mullis

  10. % Yield Practice • For the reaction SO3 + H2O ® H2SO4, calculate the percent yield if 500. g of sulfur trioxide react with excess water to produce 575 g of sulfuric acid. • For the reaction 2KClO3® 2KCl + 3O2, calculate the percent yield if 10. g of potassium chlorate is heated to produce 2.5 g of oxygen. • For the reaction 2KClO3® 2KCl + 3O2, calculate the percent yield if 10. g of potassium chlorate is heated to produce 3.0 g of potassium chlorate. • 93.9% • 65 % • 49% Mullis

  11. % Yield Practice 2 • For the reaction SO3 + H2O ® H2SO4, calculate the percent yield if 500. g of sulfur trioxide react with excess water to produce 475 g of sulfuric acid. • For the reaction 2HCl + Mg(OH)2® MgCl2 + 2H2O , calculate the percent yield if 3.65 g of HCl reacts to produce 3.0 g of magnesium chloride. • For the reaction 2HCl + Mg(OH)2® MgCl2 + 2H2O , calculate the percent yield if 3.65 g of HCl reacts to produce 4.0 g of magnesium chloride. • 77.6% • 63% • 84% Mullis

  12. Limiting Reactants A limiting reactant is the one that limits the amount of product that can be obtained. An excess reactant is one that is not used up completely in a reaction. 6 bottles 8 corks Limiting reactant: bottles Excess reactant: corks Mullis

  13. Sample Problem Zinc citrate, Zn3(C6H5O7)2 is found in some toothpastes. It is synthesized by the reaction of zinc carbonate with citric acid. 3ZnCO3+2C6H8O7 Zn3(C6H5O7)2+3H2O + CO2 With 1 mole each ofZnCO3 andC6H8O7, which is the limiting reactant? • 1 mol ZnCO3x 2mole C6H8O7=0.67 mol C6H8O7 needed 3 mole ZnCO3 • 1 mol C6H8O7 x3 mole ZnCO3= 1.5 mol ZnCO3 needed 2mole C6H8O7 Mullis

  14. Sample Problem, cont. 3ZnCO3+2C6H8O7 Zn3(C6H5O7)2+3H2O + CO2 If there is 6 mol ZnCO3 and10 molC6H8O7, which is the reactant in excess? • 6 mol ZnCO3 x 2mole C6H8O7= 4 mol C6H8O7 needed 3 mole ZnCO3 • 10 mol C6H8O7 x3 mole ZnCO3= 15 mol ZnCO3 needed 2mole C6H8O7 C6H8O7 is in excess. Mullis

  15. Limiting Reactants, Sample Problems 1 • Benzene and chlorine react to form chlorobenzene and HCl: C6H6 (l)+ Cl2 (g) C6H5Cl(s) + HCl(g) • What is the limiting reactant if 2 moles C6H6 are mixed with 1 mole Cl2? • What is the limiting reactant if 0.5 moles C6H6 are mixed with 0.75 moles Cl2? • If Cl2 is provided in excess, how much C6H5Cl is formed when 36.8 g of C6H6 is added? Mullis

  16. Limiting reactants using mass Ca3(PO4)2 + 3H2SO4 3CaSO4 + 2H3PO4 If 250 g of Ca3(PO4)2 react with 3 mol of H2SO4, will 3 mol of CaSO4 be formed? 250 g Ca3(PO4)2 x 1 mol = 0.81 mol Ca3(PO4)2 310.18 g Molar mass Ca3(PO4)2 = (3 x 40.08) + (2 x 30.97)+ (8 x 16) = 310.18 g/mol 0.81 mol Ca3(PO4)2 x 3 mol CaSO4 = 2.43molCaSO4 1 mol Ca3(PO4)2 No- we have only 2.43 mol CaSO4. Mullis

  17. Finding remaining Amounts of excess Balanced equations can only be used to calculate the amount of substances that react. To find how much is left over, or how much DOES NOT react, you must take the difference between starting and the amount that reacts. 8 Zn(s) + S8(s) 8 ZnS(s) If 2.00 mol Zn are heated with 1.00 mol S8, which is the limiting reactant? 2 mol Zn x 1 mol S8 = 0.25 mol S8 1 mol S8 x 8 mol Zn = 8 mol Zn 8 mol Zn 1 mol S8 How many moles of excess reactant remain? 1.00 mol S8 - 0.25 mol S8 =0.75 mol S8 Mullis

  18. Limiting Reactants, Sample Problems 2 • Methanol, CH3OH, is synthesized by the reaction of hydrogen and carbon monoxide. a. If 500 mol of CO and 750 mol of H2 are present, which is the limiting reactant? b. How many moles of the excess reactant remain unchanged? c. How moles of CH3OH are formed? 2. In the formation of zinc citrate, how many moles of Zn3(C6H5O7)2would be produced with 6 mol ZnCO3 and 10 mol C6H8O7? See your notes and use the equation and the limiting reactant to solve. Mullis

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