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Stoichiometry and Calculations from Chemical Equations

Learn about the Mole-Ratio Method, Mole-Mole Calculations, Mass-Mass Calculations, Limiting-Reactant, and Percent Yield. Understand how to convert moles and masses of substances using balanced equations. Practice with examples.

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Stoichiometry and Calculations from Chemical Equations

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  1. Chapter 9Calculations from Chemical Equations(Stoichiometry) Objectives: The Mole-Ratio Method Mole-Mole Calculations Mole-Mass Calculations Mass-Mass Calculations Limiting-Reactant and Percent Yield Calculations

  2. A Short Review • Molar Mass • Sum of the atomic masses of all atoms in a molecule • Molecules and Moles • A molecule is the smallest unit of a substance • A mole is Avogadro’s number (6.022 x 1023) molecules of that substance • Balanced Equations • Equations must be balanced • Number in front of a formula in a balanced chemical equation represent the number of moles of that substance

  3. The Mole-Ratio Method • Stoichiometry • quantitative relationships among reactants and products • Mole ratio • Ratio between the number of moles of any two species involved in a chemical reaction • 2H2 + O2 2H2O • Six mole ratios can be written: 2 mol H2 1 mol O2 2 mol H2 2 mol H2O 1 mol O2 2 mol H2 1 mol O2 2 mol H2O 2 mol H2O 2 mol H2 2 mol H2O 1 mol O2

  4. The Mole-Ratio Method • Use the mole ratio to convert number of moles of one substance to number of moles of another substance • How many moles of H2O can be obtained from 4.0 moles of O2? 4.0 mol O2 2 mol H2O 1 mol O2 = 8.0 mol H2O

  5. The Mole-Ratio Method • Three basic steps: • Convert the quantity of starting substance to moles (if it is not given in moles) • Convert the moles of starting substance to moles of desired substance • Convert the moles of desired substance to the units specified in the problem Grams of B Grams of A Moles of A Moles of B Atoms or Molecules hof A Atoms or Molecules of A

  6. Quantity of Given (mass, atoms, molecules) Quantity of Unknown (mass, atoms, molecules) Convert to Moles Convert to desired units Moles Of Given Find molar ratio Moles Of Unknown

  7. Mole-Mole Calculations • Quantity of given substance in moles • Quantity of desired substance requested in moles • How many moles of carbon dioxide will be produced by the complete reaction of 2.0 mol of glucose (C6H12O6) according to the following reaction? • C6H12O6 + 6O2 6CO2 + 6H2O 2.0 mol C6H12O6 6 mol CO2 1 mol C6H12O6 = 12 mol CO2

  8. Mole-Mole Calculations N2H4 + 2H2O2 N2 + 4H2O • If you have 3 moles of N2H4 how many moles of N2 will you produce? 3 mol N2H4 1 mol N2 = 1 mol N2H4 • How many molecules of H2O2 are needed to produce 3 moles of H2O? 3 mol H2O 2 mol H2O2 6.022 X 1023 molecules 4 mol H2O 1 mol H2O2 3 mol N2 = 9 x 1023 molecules H2O2

  9. Mole-Mass Calculations • Calculate the mass of one substance when given moles of another >> OR << • Calculate the moles of one substance when given mass of another • Mass of substance 1 to moles of substance 1 to moles of substance 2 (using mole ratio)

  10. Mole-Mass Calculations 2KClO3 2KCl + 3O2 • How many grams of potassium chlorate are needed to produce 5 moles of oxygen? • How many moles of potassium chloride can be produced from 100.0 g of potassium chlorate?

  11. Mass-Mass Calculations • Calculate the mass of one substance when given the mass of another • Mass of substance 1  moles of substance 1  moles of substance 2  mass of substance 2

  12. Mass-Mass Calculations 2AgNO3 + H2S  Ag2S + 2HNO3 • How many grams of silver nitrate are required to produce 250.0 grams of silver sulfide? • How many grams of nitric acid will be produced if 325 grams of H2S is used?

  13. Mass-Mass Calculations • What mass of water is produced by the complete combustion of 225.0 g of butane (C4H10)? • Write reaction • Balance • Solve problem • 2C4H10 + 13O2 8CO2 + 10H2O

  14. Part One Homework • Paired Exercises # 3-19 odd • Additional Exercises #33 & 37 • Enjoy your Thanksgiving!!! • See you Tuesday, December 2nd • Be prepared for a QUIZ over this information! 

  15. Limiting Reactants • Often, quantities of reactants are not perfect • One may be left over • Amount of product formed depends on reactant that is not in excess • Limiting reactant • One batch of chocolate chip cookies requires 4 eggs, 4 cups of flour, and 12 oz. of chocolate chips and produces 45 cookies. • How many batches can be made from 1 dozen eggs, 15 cups of flour, and 36 oz. of chocolate chips? • How many cookies will be produced from 8 eggs, 4 cups of flour, and 24 oz. of chocolate chips? 3 batches (eggs are limiting reactant) Flour is limiting reactant, only 1 batch, so 45 cookies

  16. Limiting Reactants H2 + Cl2 2HCl • How many grams of hydrogen chloride can be produced from 0.490 g of hydrogen and 50.0 g of chlorine? • Calculate mass-mass for BOTH reactants • The one with the LEAST product is your limiting reactant (and thus your answer)

  17. Limiting Reactants • If 36.5 g HCl is reacted with 85.6 g of Ba(OH)2 which is the limiting reactant? How much BaCl2 will be produced? • Barium hydroxide is the limiting reactant • 104 g BaCl2 will be produced

  18. Percent Yield • Thus far, quantities represent maximum yield (100%) • Actual yield in lab is not often 100% • Side reactions • Many reactions reversible • Poor lab skills • Expected (theoretical) yield • calculated amount of product that can be obtained • Actual yield • Amount actually obtained

  19. Percent Yield • Percent yield • Ratio of actual yield to theoretical yield Actual yield x 100 = percent yield Expected yield • Determine the percent yield for the reaction between 2.80 g Al(NO3)3 and excess NaOH if 0.966 g Al(OH)3 is recovered. (your second product is NaNO3

  20. Percent Yield • A student places an iron nail with a mass of 2.32 g into a flask of CuSO4. The nail reacts completely, leaving a quantity of copper metal in the bottom of the flask. The student finds the mass of the recovered copper to be 2.51 g. The equation for this reaction is: Fe + CuSO4 FeSO4 + Cu • What is the expected yield? • What is the percent yield?

  21. Percent Yield • When octane (C8H18) is burned in oxygen, carbon dioxide and water are produced. If 320 g of octane are burned and 392 g of water are recovered, what is the percent yield of the experiment? • First, write and balance the equation • Then, answer the question

  22. Percent Yield • 4Al + 3O2 2Al2O3 • If 0.25 mol Al and 0.40 mol O2 are reacted, which is the limiting reactant? • If a student reports a percent yield of 75.5%, how much product (in grams) did she recover?

  23. Part Two Homework • Questions # 1 & 2 • Paired Exercises # 21, 23, 27 & 29 • Additional Exercises # 31 & 41 • Be prepared for TEST on Thursday • Last EXAM of the quarter… • Remember, EC due December 9th

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