Stoichiometry

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Stoichiometry
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Stoichiometry

• The study of quantitative relationships between
  amounts of reactants and products. Based on the
  Law of conservation of Massnothing lost or
  gained in a chemical reaction
• Stoichiometry depends on a balanced chemical
  equation if equation is not written correctly
  and balanced , NOTHING can be determined

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Interpreting Equations

• Since molecules and formula units are the basic
  reacting pieces, moles must be used to transfer
  information between reactants and products. The
  balanced equation should then be considered in
  terms of moles
• 4 Fe 3 O2 ? 2 Fe2O3
• 4moles 3 moles 2 moles
• C3H8 5 O2 ? 3 CO2 4 H2O
• 1 mole 5 moles 3 moles 4 moles

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Examplesp3561
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Mole Ratios

• Every relationship between reactants and products
  can be reduced to a ratio of moles from the
  balanced equation
• 2 Al 3 Br2 ? 2 AlBr3
• Ratios can be made for each 3 Br2 or
  2 AlBr3
• 2 Al 2 Al
• 2 Al or 2 AlBr3 2
  Al or 3 Br2
• 3 Br2 3 Br2 2 AlBr3 2
  AlBr3
• Each will be used when needed to change between
  substances in stoichiometry

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Examplesp3572
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Homework 12a

• p. 357 3
• p378 ff 51-53, 54, 56, 58, 60

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Stoichiometric Calculations

• From moles to molesfollow the ratios
• 2 K 2 H2O ? 2 KOH H2
• How many moles H2 from 0.0400 moles K ?
• 0.0400 moles K 1 mole H2 0.0200
  moles H2
• 2 moles K
• moles given mole ratio

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Examplep 359
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Moles to Mass

• Involves one more stepchange moles of product to
  mass using molar mass
• 2 Na Cl2 ? 2 NaCl
• What mass of NaCl from 1.25 moles Cl2?

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Mass to Mass

• Full Regular stoichiometric calculation involves
  first change given mass to moles, switch
  substances with the mole ratio, then change back
  to mass of new substance with its molar mass.
• NH4NO3 ? N2O 2 H2O
• What mass of H2O from 25.0 g NH4NO3 ?

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Stoichiometry

• Calculations within reactions-- Bridge Method
• 1. Find reactants and products and write a
  balanced reaction
• 2. Place amount of given substance at top left
  of bridge
• 3. Convert the given to moles if necessary in
  the next span of the bridge
• 4. Change from moles of given substance to
  moles of desired substance in the nest span,
  using coefficients from the balanced equation
• 5. Change from moles to the desired unit in the
  final span of the bridge

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Mole Changer

• PARTICLES??MOLES??MASS (grams)

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Examples

• One way of producing O2(g) involves the
  decomposition of potassium chlorate into
  potassium chloride and oxygen gas. A 25.5 g
  sample of Potassium chlorate is decomposed. How
  many moles of O2(g) are produced?

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Examples

• How many grams of each reactant are needed to
  produce 15 grams of iron form the following
  reaction? Fe2O3(s) Al(s) Fe(s) Al2O3(s)

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Examples

• K2PtCl4(aq) NH3(aq) Pt(NH3)2Cl2 (s) KCl(aq)
• what mass of Pt(NH3)2Cl2 can be produced from 65
  g of K2PtCl4 ?
• How much KCl will be produced?

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Examples
K2PtCl4(aq) NH3(aq) Pt(NH3)2Cl2 (s)
KCl(aq) How much from 65 grams of NH3?
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Homework 12b

• p. 359 10
• p. 360 12
• p. 362 13, 14
• p. 379 ff 62, 64, 72, 74, 75

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Limiting Reagent

• Reactant that determines the amount of product
  formed.
• The one you run out of first.
• Makes the least product.

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Limiting Reagent

• To determine the limiting reagent requires that
  you do two stoichiometry problems.
• Figure out how much product each reactant makes.
• The one that makes the least is the limiting
  reagent.

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Example

• Ammonia is produced by the following
  reaction N2 H2 NH3
• What mass of ammonia can be produced from a
  mixture of 100. g N2 and 500. g H2 ?
• How much unreacted material remains?

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Excess Reagent / Yield

• The reactant you dont run out of.
• The amount of stuff you make is the yield.
• The theoretical yield is the amount you would
  make if everything went perfect.
• The actual yield is what you make in the lab.

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Percent Yield

• yield Actual x 100
  Theoretical
• yield what you got x 100
  what you could have got

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Examples

• Aluminum burns in bromine producing aluminum
  bromide. In a laboratory 6.0 g of aluminum reacts
  with excess bromine. 50.3 g of aluminum bromide
  are produced. What are the three types of yield.

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Examples

• Years of experience have proven that the percent
  yield for the following reaction is 74.3 Hg
  Br2 HgBr2
• If 10.0 g of Hg and 9.00 g of Br2 are reacted,
  how much HgBr2 will be produced?

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Examples
Hg Br2 HgBr2 If the reaction did go to
completion, how much excess reagent would be
left? (10.0 g of Hg and 9.00 g of Br2 )
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Examples

• Commercial brass is an alloy of Cu and Zn. It
  reacts with HCl by the following reaction
• Zn(s) 2HCl(aq) ZnCl2 (aq) H2(g) Cu
  does not react. When 0.5065 g of brass is reacted
  with excess HCl, 0.0985 g of ZnCl2 are eventually
  isolated. What is the composition of the brass?

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Homework 12c

• p. 368 20
• p. 372 28
• p. 380ff 78, 82, 84, 87