Stoichiometry

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Stoichiometry

• Chapter 9

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Stoichiometry
stoicheion element
metron measure
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Reaction Stoichiometry

• The study of the mass relationships between
  reactants and products in a chemical reaction.

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Mg(s) 2 HCl(aq) ? MgCl2(aq) H2(g)

• If you know the amount of one substance you can
  determine the amounts of all the other
  substances.
• How many moles of H2 are obtained from 2 mole of
  HCl?
• How many moles of Mg will react with 2 mole of
  HCl?
• Double the number of moles of reactants, how many
  moles of product would we have?
• How would you convert the moles of these
  substances to masses?

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To solve stoichiometry problems you must have

• A balanced chemical equation to determine the
  correct mole ratios

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Mole Ratio

• A conversion factor that relates the amounts in
  moles of any two substances involved in a
  chemical reaction.

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2 H2O ? 2 H2 O2

• 2 mole H2 and 1 mole O2
• 1 mole O2 2 mole H2
• 2 mole H2 and 2 mole H2O
• 2 mole H2O 2 mole H2
• 1 mole O2 and 2 mole H2O
• 2 mole H2O 1 mole O2

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Mole/Mole Problems

• A balanced chemical equation
• Determine the mole ratio
• Set-Up problem.start with the given

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Example 1

• The elements lithium and oxygen react to form
  lithium oxide. How many mole of lithium oxide
  will form if 2 mole of lithium react?

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Li(s) O2(g) ? Li2O(s)

• Balance equation
• 4 Li O2 ? 2 Li2O
• 2 moles mole?
• Mole ratio of lithium to lithium oxide
• 4 mole lithium to 2 mole lithium oxide
• Set-Up problem, start with given.

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• 2 mole Li 2 mole Li2O
• 4 mole Li
• Answer 1 mole Li2O

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Problem 2

• Hydrogen peroxide decomposes to form water and
  oxygen gas. How many moles of oxygen will result
  from the decomposition of 5.0 mole of H2O2?

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H2O2 ? H2O O2

• Balance equation
• 2 H2O2 ? 2 H2O O2
• 5.0 mole mole?
• Mole ration of hydrogen peroxide oxygen
• 2 mole H2O 2 to 1 mole O2
• Set-up problem starting with the given.

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• 5.O mole H2O2 1 mole O2
• 2 mole H2O2
• Answer 2.5 mole of O2

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Problem 3

• Ammonia, NH3 is widely used as a fertilizer and
  in many household cleaners. How many moles of
  ammonia are produced when 6 mole of hydrogen gas
  react with an excess of nitrogen gas?

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N2 H2 ? NH3

• Balance equation
• N2 3 H2 ? 2 NH3
• 6 mole mole?
• 2. Determine mole ratio of hydrogen to ammonia
  
• 3 mole hydrogen to 2 mole ammonia
• Set-up problem starting with the given.

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• 6 mole H2 2 mole NH3
• 3 mole H2
• Answer 4 mole NH3

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Mass/Mole and Mole/Mass Problems

• Must have 3 pieces of information
• Balanced chemical equation
• Mole ratio
• Molar mass

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Problem 1

• What mass in grams of magnesium oxide is produced
  from 2.00 mole of magnesium?

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• Mg O2 ? MgO
• 2 Mg O2 ? 2 MgO
• 2.00 mole grams?
• Molar Mass of MgO 40.31 g/mole
• 2.00 mole Mg 2 mole MgO 40.31 g MgO
• 2 mole Mg 1 mole MgO

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• Answer 80.62 g MgO 80.6 g MgO

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Problem 2

• What mass in grams of oxygen combine with 2.00
  mole of magnesium?

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• 2 Mg O2 ? 2 MgO
• 2.00 mole grams?
• Molar mass of oxygen 32.00 g/mole
• 2.00 mole Mg 1 mole O2 32.00 g O2
• 2 mole Mg 1 mole O2
• Answer 32.0 g O2

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Problem 3

• How many mol of HgO are needed to produce 125
  grams of oxygen?

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• HgO ? Hg O2
• 2 HgO ? 2 Hg O2
• moles? 125 grams
• Molar mass of oxygen 32.00 g/mole
• 125 g O2 1 mole O2 2 mole HgO
• 32.00 g O2 1 mole O2
• Answer 7.81 mole HgO

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Mass/Mass Problems

• Balanced equation
• Mole ratio
• Molar Masses

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Problem 1

• Tin (II) fluoride, SnF2 is used in toothpaste.
  How many grams of tin (II) fluoride are produced
  from the reaction of 30.0 g of aqueous
  hydrofluoric acid with Sn?
• Sn(s) HF(aq) ?
• SnF2 H2(g)

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• How many grams of tin (II) fluoride are produced
  from the reaction of 30.0 g of HF with Sn?
• Sn(s) 2 HF(aq) ? SnF2 H2(g)
  30.0 g grams?
• Molar masses of HF and SnF2?
• HF 20.01 g/mole
• SnF2 156.71 g/mole

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How many grams of SnF2 are produced from the
reaction of 30.0 g of HF with Sn?

• 30.0gHF 1 mole HF 1 mole SnF2 156.71g SnF2
  
• 20.01g HF 2 mole HF 1 mole
  SnF2
• Answer 117 g SnF2

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Problem 2

• Nitrous oxide, N2O a.k.a. laughing gas is used as
  an anesthetic in dentistry. How many grams of
  NH4NO3 is required to form 33.0g 33.0g N2O? (the
  other product of this decomposition reaction is
  water)
• NH4NO3 ? N2O H2O

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NH4NO3 ? N2O 2 H2O grams?
33.0g

• Molar Mass of NH4NO3 80.06 g/mole
• Molar Mass of N2O 44.02 g/mole

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NH4NO3 ? N2O 2 H2O grams?
33.0g

• 33.0g N2O 1 mole N2O 1 mole NH4NO3 80.06 g
  NH4NO3
• 44.02gN2O 1 mole N2O
  1 mole NH4NO3
• Answer 60.0 g NH4NO3

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Stoichiometry

• 9-3

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Limiting Excess Reactants

• Limiting Reactant the reactant is used up first
  in a chemical reaction.
• Excess Reactant the reactant that is not totally
  used up in a chemical reaction.

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Identify the limiting excess reactants

• Suppose you have 10 slices of bread and four
  slices of cheese. How many sandwiches can you
  make? (Assume that the cheese is thick sliced)

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• What is the limiting reactant?
• What is the excess reactant?

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Musical Chairs

• In the game Musical Chairs, what is the limiting
  reagent and what is the excess reagent?

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C (s) O2 (g) ? CO2 (g)

• Suppose in this reaction we have 3 C atoms and 6
  molecules of O2 (g).

3 C atoms 6 O2 molecules ? 3 CO2 molecules 3
O2 molecules
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Limiting Reactant Problems

• Balanced chemical equation.
• Determine the number of moles of each reactant.
• Divide each number of moles of each reactant by
  the coefficient in the balanced equation. The
  smallest number is the limiting reactant.
• Proceed with solving problem.

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Limiting Reactant Problem 1

• SiO2 4 HF ? SiF4 2 H2O
• If 2.0 mol of HF are exposed to 4.5 mol of SiO2,
  which is the limiting reactant?

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SiO2 4 HF ? SiF4 2 H2O

• If 2.0 mol of HF are exposed to 4.5 mol of SiO2,
  which is the limiting reactant?
• HF 2.0 mol 4 0.5
• SiO2 4.5 mol 1 4.5
• Limiting Reactant?
• HF

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Limiting Reactant Problem 2

• If 20.5 g of chlorine is reacted with 20.5 g of
  sodium, which is the limiting reactant? How much
  salt is formed?
• Cl2 Na ? NaCl
• Cl2 2 Na ? 2 NaCl

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Cl2 2 Na ? 2 NaCl

• If 20.5 g of chlorine is reacted with 20.5 g of
  sodium, which is the limiting reactant?
• 20.5 g Cl2 x 1 mol Cl2 0.289 mol
• 70.9 g
• 20.5 g Na x 1 mol Na 0.892 mol
• 22.99 g
• Cl2 0.289 1 0.289
• Na 0.892 2 0.445

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How much salt is formed?

• Limiting Reactant?
• Chlorine
• 0.289 mol Cl2 x 2 mol NaCl x 58.44 g NaCl
• 1 mol Cl2 1 mol
  NaCl
• 33.8 g NaCl

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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 measured amount of product
  obtained from a reaction.

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• Percent Yield
• Actual yield x 100
• Theoretical yield

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Example 1

• Chlorobenzene is used in the production of many
  important chemicals, such as aspirin, dyes, and
  disinfectants. One industrial method of preparing
  chlorobenzene is to react benzene, with chlorine,
  as represented in the following reaction
• C6H6 (l) Cl2 (g) ? C6H5Cl (s) HCl (g)

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• When 36.8g of C6H6 react with an excess of Cl2,
  the actual yield of C6H5Cl is 38.8g. What is the
  percent yield of C6H5Cl?
• C6H6 (l) Cl2 (g) ? C6H5Cl (s) HCl (g)

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Step 1

• Do a mass-mass calculation to find the
  theoretical yield.
• Answer 53.0 g

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Step 2

• Use percent yield equation
• 38.8g x 100
• 53.0g
• Answer 73.2

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Student Problems

• 1.) If 75.0g of CO react to produce 68.4g of
  methanol, what is the percent yield of methanol?
• CO (g) H2 (g) ? CH3OH (l) (unbalanced)
• 2.) If 1.85g of Al react and the percent yield of
  Cu is 56.6, what mass of Cu is produced?
• Al (s) CuSO4 (aq) ? Al2 (SO4)3 (aq) Cu (s)
  (unbalanced)

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Answers

• 79.8
• 3.70 g