Chapter 9: Chemical quantities

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Chapter 9 Chemical quantities
Chemistry 1020 Interpretive chemistry Andy
Aspaas, Instructor
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Interpreting chemical equations

• The coefficients in a chemical equation indicate
  relative numbers of molecules involved in the
  equation
• 2H2(g) O2(g) ? 2H2O(g)
• This equation can be interpreted many ways
• 2 molecules H2 gas react with 1 molecule O2 gas
  to form 2 molecules H2O gas
• 2 dozen molecules H2 gas react with 1 dozen
  molecules O2 gas to form 2 dozen molecules H2O
  gas
• 2 mol H2 gas react with 1 mol O2 gas to form 2
  mol H2O gas

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Mole-mole relationships in reactions

• 2H2(g) O2(g) ? 2H2O(g)
• If you want to produce 15.7 mol H2O, how many
  moles of O2 gas are required in this reaction?
• Use the coefficients in the balanced chemical
  reaction to convert between moles of any
  components in a reaction
• Mole ratio 1 mol O2(g) for every 2 mol H2O(g)
  from the equation
• Use that mole ratio as a dimensional analysis
  conversion factor

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Mass calculations

• In chemistry, number of moles is the only unit
  that can be used to convert between quantities of
  constituents in a chemical reaction
• Stoichiometry conversion of masses of different
  reactants and products in a chemical reaction
• If youre given the mass of a reactant or
  product, you must first convert mass to moles
• If youre looking to find the mass of a reactant
  or product, you must first find the moles, and
  then convert to mass

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Practice mass calculation problems

• CO(g) 2H2(g) ? CH3OH(l)
• What masses of carbon monoxide gas and hydrogen
  gas are required to produce 6.0 kg of methanol?
• LiOH(s) CO2(g) ? Li2CO3(s) H2O(l)
• What mass of CO2 gas can 1.00 kg of lithium
  hydroxide react with?

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Limiting reagent

• Whenever you are given quantities of more than
  one reactant, you must decide which will be
  consumed first (the limiting reactant)
• The cheese sandwich analogy
• Decide quantity of products that each individual
  reactant is able to produce
• The reactant that produces the smallest quantity
  of product is the limiting reactant

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Limiting reactant

• Convert each reactant to moles
• Use mole ratios to convert each to moles of
  product
• Reactant which produces smallest value of product
  moles is limiting reactant
• Use limiting reactant to calculate product moles
  and finally product mass

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Limiting reactant practice

• N2(g) 3H2(g) ? 2NH3(g)
• Say 25.0 g nitrogen gas and 5 g hydrogen gas are
  reacted to form ammonia. What mass of ammonia can
  be produced?
• Convert reactants to moles
• Convert each to moles products
• Identify limiting reagent
• Product moles from limiting reagent
• Convert to mass of product

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Percent yield

• All calculations so far have given theoretical
  yields of products
• The maximum amount of product that can be formed,
  assuming all of the limiting reagent reacts, and
  all product can be collected
• In reality, actual yield is almost always lower
• Percent yield measure of experimental efficiency
• (actual yield) / (theoretical yield) x 100