Unit 6 Stoichiometry

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Unit 6 Stoichiometry
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What Exactly Is Stoichiometry?

• Composition stoich deals with mass
  relationships of elements in compounds (review Ch
  3)
• Reaction stoich deals with relationships
  between reactants and products

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What Youve Learned So Far

• Unit Conversions
• Naming Chemical Compounds
• Writing Chemical Formulas
• The Mole
• Writing Chemical Equations
• Balancing Chemical Equations

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Mole Ratios

• Conversion factor relating amount of moles of any
  two substances

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Example
or
or
or
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Molar Mass Ratios

• Al2O3 101.96 g/mol
• Al 26.98 g/mol
• O2 32.00 g/mol

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Solving Stoichiometry Problems

• Must have
• Correct molecular formulas
• Balanced chemical equation

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Steps to Solving Problems

1. Write a balanced chemical equation
2. Identify information given in problem
3. Pick appropriate molar mass or mole ratio
4. Multiply numerators
5. Multiply denominators

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Reaction Stoichiometry

• Mole-Mole
• Mole-Mass or Mass-Mole
• Mass-Mass

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Type 1

• Given quantity and unknown quantity in moles
• Quantity given (in mol) ?
• Quantity unknown (in mol)

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Stoichiometry Examples Type 1 (mol-mol)

• Example 1 The combustion of 2.19 moles propane
  (C3H8) produces how many moles carbon dioxide?
• Example 2 41.8 mol lithium hydroxide is combined
  with carbon dioxide to produce lithium carbonate
  and liquid water. How much water is produced from
  this reaction?

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Type 2

• Given amount is in moles and unknown is mass in
  grams
• Amount given (in mol) ?
• amount unknown (in mol) ?
• mass unknown (in g)

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Stoichiometry Examples Type 2 (mol-mass)

• Example 3
• In photosynthesis, plants use energy from the sun
  to produce glucose C6H12O6, and oxygen from the
  reaction of carbon dioxide and water. What mass,
  in grams, of glucose is produced when 3.00 mol of
  water react with carbon dioxide?

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Type 3

• Given is amount in grams and unknown is amount in
  moles
• mass given (in g) ?
• amount given (in mol) ?
• amount unknown (in mol)

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Stoichiometry Examples Type 3 (mass-mol)

• NaHCO3 and Mg(OH)2 are both used as antacids
  which is more effective per gram?
• Example 4 1.00 g NaHCO3 reacts with hydrochloric
  acid to produce sodium chloride, water, and
  carbon dioxide
• Example 5 1.00 g Mg(OH)2 reacts with
  hydrochloric acid to produce magnesium chloride
  and water

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Stoichiometry ExamplesType 3 (mol-mass)

• Example 6
• What mass of carbon dioxide, in grams, is needed
  to react with 3.50 mol of water in the
  photosynthetic reaction?

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Objective

• III.A.3(i) Use chemical equations to perform
  basic mole-mole, mass-mass, and mass-mole
  computations for chemical reactions

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Type 4

• Given is mass in grams and unknown is mass in
  grams
• mass given (in g) ?
• amount given (in mol) ?
• amount unknown (in mol) ?
• mass unknown (in g)

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Stoichiometry ExamplesType 4 (mass-mass)

• Example 7
• Tin (II) fluoride is used in some toothpastes.
  It is made by the reaction of tin with hydrogen
  fluoride according to the following equation.
• Sn(s) 2HF ? SnF2(s) H2(g)
• How many grams of SnF2 are produced from the
  reaction of 30.00 g of HF with Sn?

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Limiting Reagents

• III.A.3(j) identify limiting reagents and use
  this information when solving stoichiometry
  problems

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What is a limiting reagent?

• The reactant that is completely used up first in
  a chemical reaction

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How to Find the Limiting Reagent

1. Analyze the question
2. Identify the given information
3. Write a balanced chemical reaction
4. Convert given information of Reactant 1 to
   Product A
5. Convert given information of Reactant 2 to
   Product A

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Theoretical Yield

• III.A.3(k) compute theoretical yield, actual
  (experimental) yield, and percent yield

III.A.3(l) calculate percent error and analyze
experimental errors that affect percent error
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Theoretical Yield

• Using the limiting reagent to calculate the
  amount of product produced is called the
  theoretical yield
• This is how much product could be made if the
  limiting reactant was totally consumed

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Experimental Yield

• Also called the actual yield
• The amount of product actually collected in the
  laboratory is called the experimental yield

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Percent Yield
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Example Limiting Reactant

• Aluminum reacts with chlorine gas to form
  aluminum chloride.
• In a certain experiment, 10.0 g of aluminum is
  reacted with 35.0 g of chlorine gas.
• What mass of aluminum chloride will be produced,
  assuming a complete reaction?
• What mass of which reactant is left after the
  reaction?