STOICHIOMETRY

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STOICHIOMETRY

• USING THE REACTION EQUATION LIKE A RECIPE

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USING EQUATIONS

• Nearly everything we use is manufactured from
  chemicals.
• Soaps, shampoos, conditioners, cds, cosmetics,
  medications, and clothes.
• For a manufacturer to make a profit the cost of
  making any of these items cant be more than the
  money paid for them.
• Chemical processes carried out in industry must
  be economical, this is where balanced equations
  help.

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USING EQUATIONS

• Equations are a chemists recipe.
• Eqs tell chemists what amounts of reactants to
  mix and what amounts of products to expect.
• When you know the quantity of one substance in a
  rxn, you can calculate the quantity of any other
  substance consumed or created in the rxn.
• Quantity meaning the amount of a substance in
  grams, liters, molecules, or moles.

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USING EQUATIONS

• The calculation of quantities in chemical
  reactions is called stoichiometry.
• When you bake cookies you probably use a recipe.
• A cookie recipe tells you the amounts of
  ingredients to mix together to make a certain
  number of cookies.
• If you need more cookies than the yield of the
  recipe, the amounts of ingredients can be doubled
  or tripled.

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USING EQUATIONS

• In a way, a cookie recipe provides the same kind
  of information that a balanced chemical equ.
  provides
• Ingredients are the reactants
• Cookies are the products.

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USING EQUATIONS

• Imagine you are in charge of manu-facturing for
  Rugged Rider Bicycle Company.
• The business plan for Rugged Rider requires the
  production of 128 custom-made bikes each day.
• One of your responsibilities is to be sure that
  there are enough parts available at the start of
  each day.

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USING EQUATIONS

• Assume that the major components of the bike are
  the frame (F), the seat (S), the wheels (W), the
  handlebars (H), and the pedals (P).
• The finished bike has a formula of FSW2HP2.
• The balanced equation for the production of 1
  bike is.

F S2WH2P ? FSW2HP2
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USING EQUATIONS

• Now in a 5 day workweek, Rugged Riders is
  scheduled to make 640 bikes. How many wheels
  should be in the plant on Monday morning to make
  these bikes?
• What do we know?
• Number of bikes 640 bikes
• 1 FSW2HP22W (balanced eqn)
• What is unknown?
• of wheels ? wheels

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• The connection between wheels and bikes is 2
  wheels per bike. We can use this information as
  a conversion factor to do the calculation.

2 W
640 FSW2HP2
1 FSW2HP2

• We can derive the same connections from a normal
  chemical rxn equation.

N2(g) 3H2(g) ? 2NH3(g)

• What can we learn from this equation?

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• Remember, The coefficients of the balanced
  chemical equation indicate the numbers of moles
  of reactants and products in a chemical reaction.
  
• 1 mole of N2 reacts with 3 moles of H2 to produce
  2 moles of NH3.
• N2 and H2 will always react to form ammonia in
  this 132 ratio of moles.
• So if you started with 10 moles of N2 it would
  take 30 moles of H2 and would produce 20 moles of
  NH3

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• Using the coefficients, from the balan-ced rxn
  equation to make connections between reactants
  and products, is the most important information
  that a rxn equation provides.
• Using this information, you can calculate the
  amounts of reactants and products.
• You can make a similar connections with mass and
  with volume at STP, because of how each relates
  to the mole

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• The mass of 1 mol of N2 molecules is 28 g the
  mass of 3 mols of H2 molecules is 6 g for a
  total mass of reactants of 34 g.
• The mass of 2 moles of NH3 molecules is 2 17g
  or 34 g.
• Reactants masses products masses
• Remember that 1 mole of any gas at STP occupies
  22.4 L of space.
• It follows that 22.4 L of N2 reacts with 67.2 L
  of H2 to form 44.8 L of Ammonia gas. (notice
  volumes dont balance)

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22.4 L
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• Using the coefficients of balanced rxn equations
  and our knowledge of mole conversions we can
  perform powerful calculations. A.K.A.
  stoichiometry.
• A balanced rxn equation is essential for all
  calculations involving amounts of reactants and
  products.
• If you know the number of moles of 1 substance,
  the balanced eqn allows you to calc. the number
  of moles of all other substances in a rxn
  equation.

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MOLE MOLE EXAMPLE

• The following rxn shows the synthesis of aluminum
  oxide.

3O2(g) 4Al(s) ? 2Al2O3(s)
3O2(g) 4Al(s) ? 2Al2O3(s)

• How many moles of aluminum are needed to form 3.7
  mol Al2O3?

Given 3.7 moles of Al2O3
Uknown ____ moles of Al
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MOLE MOLE EXAMPLE 2

• Solve for the unknown

3O2(g) 4Al(s) ? 2Al2O3(s)
4 mol Al
3.7 mol Al2O3
7.4 mol Al
2 mol Al2O3
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MASS MASS CALCULATNS

• No lab balance measures moles directly, instead
  the mass of a substance is usually measured.
• From the mass of 1 reactant or prod-uct, the mass
  of any other reactant or product in a given
  chemical equation can be calculated, provided you
  have a balanced rxn equation.
• As in mole-mole calcs, the unknown can be either
  a reactant or a product.

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MASS MASS CALCULATNS
Acetylene gas (C2H2) is produced by adding water
to calcium carbide (CaC2).
CaC2 2H2O ? C2H2 Ca(OH)2
How many grams of C2H2 are produced by adding
water to 5.00 g CaC2?
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MASS MASS CALCULATNS

• What do we know?
• Given mass 5.0 g CaC2
• Mole ratio 1 mol CaC2 1 mol C2H2
• MM of CaC2 64.0 g CaC2
• MM of C2H2 26.0g C2H2
• What are we asked for?
• grams of C2H2 produced

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MASS MASS CALCULATNS

• mass A ? moles A ? moles B ? mass B

5.0 g CaC2
1 mol CaC2
1 mol C2H2
64.0 g CaC2
1mol CaC2
26.0 g C2H2
1mol C2H2
2.30 g C2H2
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PRACTICE TIME 11-1 11-2
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• A balanced reaction equation indicates the
  relative numbers of moles of reactants and
  products.
• We can expand our stoichiometric calculations to
  include any unit of measure that is related to
  the mole.
• The given quantity can be expressed in numbers of
  particles, units of mass, or volumes of gases at
  STP.
• The problems can include mass-volume,
  volume-volume, and particle-mass calculations.

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• In any of these problems, the given quantity is
  first converted to moles.
• Then the mole ratio from the balanced eqn is used
  to convert from the moles of given to the number
  of moles of the unknown
• Then the moles of the unknown are converted to
  the units that the problem requests.
• The next slide summarizes these steps for all
  typical stoichiometric problems

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MORE MOLE EXAMPLES
How many molecules of O2 are produced when a
sample of 29.2 g of H2O is decomposed by
electrolysis according to this balanced equation
2H2O ? 2H2 O2
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MORE MOLE EXAMPLES

• What do we know?
• Mass of H2O 29.2 g H2O
• 2 mol H2O 1 mol O2 (from balanced equation)
• MM of H2O 18.0 g H2O
• 1 mol O2 6.02x1023 molecules of O2
• What are we asked for?
• molecules of O2

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• mass A ? mols A ? mols B ?
• molecules B

29.2 g H2O
1 mol H2O
1 mol O2
18.0 g H2O
2 mol H2O
6.02x1023 molecules O2
1 mol O2
4.88 x 1023 molecules O2
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MORE MOLE EXAMPLES
The last step in the production of nitric acid is
the reaction of NO2 with H2O. 3NO2H2O?2HNO3NO Ho
w many liters of NO2 must react with water to
produce 5.00x1022 molecules of NO?
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MORE MOLE EXAMPLES

• What do we know?
• Molecules NO 5.0x1022 molecules NO
• 1 mol NO 3 mol NO2 (from balanced equation)
• 1 mol NO 6.02x1023 molecules NO
• 1 mol NO2 22.4 L NO2
• What are we asked for?
• Liters of NO2

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• molecules A? mols? mols B? volume B

1 mol NO
3 mol NO2
5.0x1022 mol-ecules NO
1 mol NO
6.02x1023 mol-ecules NO
22.4 L NO2
1 mol NO2
5.58 L NO2
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Aspirin can be made from a chemical rxn between
the reactants salicylic acid and acetic
anhydride. The products of the rxn are
acetyl-salicylic acid (aspirin) and acetic acid
(vinegar). Our factory makes 125,000 100-count
bottles of Bayer Aspirin/day. Each bottle
contains 100 tablets, and each tablet contains
325mg of aspirin. How much in kgs 10 for
production problems, of each reactant must we
have in order to meet production?
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• What do we know?
• Make 125,000 aspirin bottles/day
• 100 aspirin/bottle
• 325 mg aspirin/tablet
• Mole ratio of aspirin to salicylic acid (11) and
  acetic anhydride (11)
• MM aspirin 180.11g
• MM C7H6O3 138.10g
• MM C4H6O3 102.06g
• What are we asked for?
• Mass of salicylic acid in kgs 10
• Mass of acetic anhydride in kgs 10

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100 tablets
325mg asp.
125,000 bottles
1 bottle
1 tablet
1 g
1mol asp.
1000 mg
180.16g
22,549.4 mols aspirin
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Salicylic Acid
1 mol C7H6O3
136.10g C7H6O3
22,549.4 mols aspirin
1 mol C7H6O3
1 mol asp
1 kg
3068.97 kg salicylic acid (306.897 g)
1000 g
3380 kg of salicylic acid
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Acetic Anhydride
1 mol C4H6O3
102.06g C4H6O3
22,549.4 mols aspirin
1 mol C4H6O3
1 mol asp
1 kg
2301.39 kg Acetic anhydride
230.139 kg
1000 g
2530 kg Acetic anhydride