Stoichiometry

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Stoichiometry

• What is Stoichiometry?
• Stoichiometry Calculation
• Limiting Reactants
• Percent Yield

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Stoichiometry

• The study of quantitative relationships between
  amounts of reactants used products formed by a
  chemical reaction
• Law of Conservation of Mass
• Mass of the reactants mass of the products

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Stoichiometry Balanced Equations

• 4Fe(s) 3O2(g) ? 2FeO3(s)
• Representative Particles
• Moles
• Mass??

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Conservation of mass

• 4Fe(s) 3O2(g) ? 2FeO3(s)

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Mole Ratios

• A ratio between the numbers of moles of any 2
  substances in a balanced chemical equation

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Mole Ratios

• 2Al(s) 3Br2(l) ? 2AlBr3(s)

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Using Stoichiometry

• What do we need?
• Balanced chemical reaction
• Mole ratios
• Mass-to-mole conversion

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Mole-to-mole

• 2K(s) 2H2O(l) ? 2KOH(aq) H2(g)
• We have 0.0400 g K
• How much H is formed?
• Mole ratio?
• Conversion factor?

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Mass-to-mole

• 0.0200 mol H2

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Mass-to-mass

• NH4NO3(s) ? N2O(g) H2O(g)
• We have 25.0 g of N2O
• How much H2O?

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Steps in Stoichiometry

• Balanced chemical equation
• Find moles using mass-to-mole conversion
• Find moles of unknown from the moles of known
• Find mass of unknown from moles of unknown

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Limiting Reactants

• Limits the extent of the reaction and thereby,
  determines the amount of product
• Excess reactants
• The left-over reactants

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Limiting Reactants

• All previous reactions are based on the idea that
  chemical reactions are balanced
• 3 Nitrogen molecules (6 N atoms)
• 3 Hydrogen molecules (6 H atoms)
• 2 Ammonia molecules 2 N2 molecules

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Steps

• Balanced chemical equation
• Mass to moles
• Correct mole ratio
• a. Gives limiting reactant
• Moles of LR to moles of product
• Moles of product to mass of product
• Moles of LR to moles of excess
• Moles of excess to mass of excess
• Excess available excess used

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Example

• The reaction between solid phosphorus and oxygen
  produces solid tetraphosphorus decoxide (P4O10).
• Determine the mass of tetraphosphorus decoxide
  formed if 25.0 g of phosphorus (P4) and 50.0 g of
  oxygen are combined.
• How much of the excess reactant remains after the
  reaction stops?

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P4(s) 5O2(g) ? P4O10
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How much excess??
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Percent Yield

• Theoretical Yield
• The maximum amount of product that can be
  produced from a given amount of reactant
• Actual Yield
• The amount of product actually produced when the
  chemical reaction is carried out in an
  experiment.
• Percent Yield
• The ratio of the actual yield to the theoretical
  yield expressed as a percent

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Percent Yield
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Percent Yield Steps

• Mass to mole conversion
• Mole to mole conversion
• Mole to mass conversion
• a. Give theoretical yield
• Calculate yield

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Example

• Aluminum hydroxide is often present in antacids
  to neutralize stomach acid (HCl). If 14.0 g of
  aluminum hydroxide is present in an antacid
  tablet, determine the theoretical yield of
  aluminum chloride produced when the tablet reacts
  with stomach acid. If the actual yield of
  aluminum chloride is from this tablet is 22.0 g,
  what is the percent yield?
• Al(OH)3(s) 3HCL(aq) ? AlCl3(aq) 3H2O(l)

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Al(OH)3(s) 3HCL(aq) ? AlCl3(aq) 3H2O(l)
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Stoichiometry??

• Mole ratios
• Mole-to-mole conversion
• Mole-to-mass conversion
• Mass-to-mass conversion
• Limiting Reactants
• Percent Yield