Chapter 4: Chemical Quantities

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Chapter 4 Chemical Quantities Aqueous Reactions

• CHE 123 General Chemistry I
• Dr. Jerome Williams, Ph.D.
• Saint Leo University

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Overview

• Stoichiometry
• Percent Yield

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Stoichiometry

• Stoichiometry deals with the quantity of
  materials consumed and or produced in chemical
  reactions.
• Stoichiometry deals specifically with mass-mole
  relationships between reactants and products
  found in a chemical equation.

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Stoichiometry

• Two Fundamental Principles
• Must always work from a balanced reaction.
• Axiom 3 is always foremost in out minds. When in
  doubt, convert to moles for they guide us
  everywhere we wish to go in chemistry.

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Stoichiometry

• How to Solve Stoichiometry Problems?
• Use Handy-Dandy Five Step Method
• 1. Write out balanced chemical reaction. Place
  the given and unknown quantities (with units)
  above item in reaction.
• 2. Calculate molar masses for items needed to
  solve problem. Place these below items in
  reaction.

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Stoichiometry

• Handy Dandy Five Step Method
• 3. Convert given quantity into moles (Axiom 3).
• 4. Using the balanced chemical equation (step 1),
  convert moles obtained in step 3 into moles of
  quantity you are after.
• 5. Convert moles of final quantity into the
  desired unit asked for in problem.

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Stoichiometry Concept Map
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Stoichiometry

• How many grams of water are produced from the
  combustion of 32.75 g of butane (C4H10)? The
  balanced chemical reaction for this process is
  shown below.
• 2 C4H10 13 O2 ? 8 CO2 10 H2O

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Stoichiometry

• Apply Handy Dandy Five Step Method
• Step 1 Equation is already balanced. Known and
  unknown values are placed above item in equation.
• 32.75 g ? g
• 2 C4H10 13 O2 ? 8 CO2 10 H2O

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Stoichiometry

• Apply Handy Dandy Five Step Method
• Step 2 Calculate Molar Masses for known and
  unknown. Values are placed below item in
  equation.
• C4H10 (4 x 12.01 g/mol) (10 x 1.01 g/mol)
  58.14 g/mol
• H2O (2 x 1.01 g/mol) (1 x 16.00 g/mol)
  18.02 g/mol

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Stoichiometry

• Apply Handy Dandy Five Step Method
• Step 2 Calculate Molar Masses for known and
  unknown. Values are placed below item in
  equation.
• 32.75 g ? g
• 2 C4H10 13 O2 ? 8 CO2 10 H2O
• 58.14 g/mol 18.02 g/mol

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Stoichiometry

• Apply Handy Dandy Five Step Method
• Step 3 Convert given quantity into moles (Axiom
  3)
• 32.75 g C4H10 X ( 1 mol C4H10 / 58.14 g C4H10 )
  0.56329 mol C4H10

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Stoichiometry

• Apply Handy Dandy Five Step Method
• Step 4 Using the balanced chemical equation (see
  Step 1), convert moles obtained in step 3 into
  moles of quantity you are after.
• Balanced reaction shows relationship between
  water and butane (2 C4H10 13 O2 ? 8 CO2 10
  H2O)
• 0.56329 mol C4H10 (10 moles H2O / 2 moles C4H10)
  2.81647 mol H2O

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Stoichiometry

• Apply Handy Dandy Five Step Method
• Step 5 Convert moles of final quantity into the
  desired unit asked for in problem.
• 2.81647 mol H2O x (18.02 g H2O / 1 mol H2O)
  50.75 g H2O
• Check Answer is reasonable with correct units.

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Percent Yield

• Percent yield is a measure of how completely
  reactants are turned into products.
• Most reactions do not go to completion (i.e.,
  reach 100 conversion) due to side reactions,
  transfer losses, etc.

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Percent Yield

• Percent yield is calculated using the following
  equation.
• yield (actual yield / theoretical yield) x
  100
• actual yield is amount obtained in the analysis.
• theoretical yield is amount obtained assuming 100
  complete reaction.

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A Second Example for Practice

• According to the U.S. Dept. Energy if 3.4 x 1015
  g of octane was burned in 2004, how many grams of
  carbon dioxide was released into the atmosphere
  that year?
• Use the Handy-Dandy Five Step Method to Solve
  this problem.

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Practice Problem

• Answer to Problem 1.0 x 1016 g CO2
• This value is the maximum amount of carbon
  dioxide that would be produced assuming complete
  reaction (hence, this is the theoretical yield).