Chapter 9 Calculations from Chemical Equations Stoichiometry

1
Chapter 9Calculations from Chemical
Equations(Stoichiometry)

• Objectives
• The Mole-Ratio Method
• Mole-Mole Calculations
• Mole-Mass Calculations
• Mass-Mass Calculations
• Limiting-Reactant and Percent Yield Calculations

2
A Short Review

• Molar Mass
• Sum of the atomic masses of all atoms in a
  molecule
• Molecules and Moles
• A molecule is the smallest unit of a substance
• A mole is Avogadros number (6.022 x 1023)
  molecules of that substance
• Balanced Equations
• Equations must be balanced
• Number in front of a formula in a balanced
  chemical equation represent the number of moles
  of that substance

3
The Mole-Ratio Method

• Stoichiometry
• quantitative relationships among reactants and
  products
• Mole ratio
• Ratio between the number of moles of any two
  species involved in a chemical reaction
• 2H2 O2 ? 2H2O
• Six mole ratios can be written

2 mol H2 2 mol H2O
2 mol H2 1 mol O2
1 mol O2 2 mol H2
1 mol O2 2 mol H2O
2 mol H2O 2 mol H2
2 mol H2O 1 mol O2
4
The Mole-Ratio Method

• Use the mole ratio to convert number of moles of
  one substance to number of moles of another
  substance
• How many moles of H2O can be obtained from 4.0
  moles of O2?
• 4.0 mol O2 2 mol H2O
• 1 mol O2

8.0 mol H2O
5
The Mole-Ratio Method

• Three basic steps
• Convert the quantity of starting substance to
  moles (if it is not given in moles)
• Convert the moles of starting substance to moles
  of desired substance
• Convert the moles of desired substance to the
  units specified in the problem

Grams of B
Grams of A
Moles of A
Moles of B
Atoms or Molecules hof A
Atoms or Molecules of A
6
Quantity of Given (mass, atoms, molecules)
Quantity of Unknown (mass, atoms, molecules)
Convert to Moles
Convert to desired units
Moles Of Given
Moles Of Unknown
Find molar ratio
7
Mole-Mole Calculations

• Quantity of given substance in moles
• Quantity of desired substance requested in moles
• How many moles of carbon dioxide will be produced
  by the complete reaction of 2.0 mol of glucose
  (C6H12O6) according to the following reaction?
• C6H12O6 6O2 ? 6CO2 6H2O
• 2.0 mol C6H12O6 6 mol CO2
• 1 mol C6H12O6

12 mol CO2
8
Mole-Mole Calculations

• N2H4 2H2O2 ? N2 4H2O
• If you have 3 moles of N2H4 how many moles of N2
  will you produce?
• 3 mol N2H4 1 mol N2
• 1 mol N2H4
• How many molecules of H2O2 are needed to produce
  3 moles of H2O?
• 3 mol H2O 2 mol H2O2 6.022 X 1023 molecules
• 4 mol H2O 1 mol H2O2

3 mol N2
9 x 1023 molecules H2O2
9
Mole-Mass Calculations

• Calculate the mass of one substance when given
  moles of another
• gtgt OR ltlt
• Calculate the moles of one substance when given
  mass of another
• Mass of substance 1 to moles of substance 1 to
  moles of substance 2 (using mole ratio)

10
Mole-Mass Calculations

• 2KClO3 ? 2KCl 3O2
• How many grams of potassium chlorate are needed
  to produce 5 moles of oxygen?
• How many moles of potassium chloride can be
  produced from 100.0 g of potassium chlorate?

11
Mass-Mass Calculations

• Calculate the mass of one substance when given
  the mass of another
• Mass of substance 1 ? moles of substance 1 ?
  moles of substance 2 ? mass of substance 2

12
Mass-Mass Calculations

• 2AgNO3 H2S ? Ag2S 2HNO3
• How many grams of silver nitrate are required to
  produce 250.0 grams of silver sulfide?
• How many grams of nitric acid will be produced if
  325 grams of H2S is used?

13
Mass-Mass Calculations

• What mass of water is produced by the complete
  combustion of 225.0 g of butane (C4H10)?
• Write reaction
• Balance
• Solve problem
• 2C4H10 13O2 ? 8CO2 10H2O

14
Part One Homework

• Paired Exercises 3-19 odd
• Additional Exercises 33 37
• Enjoy your Thanksgiving!!!
• See you Tuesday, December 2nd
• Be prepared for a QUIZ over this information! ?

15
Limiting Reactants

• Often, quantities of reactants are not perfect
• One may be left over
• Amount of product formed depends on reactant that
  is not in excess
• Limiting reactant
• One batch of chocolate chip cookies requires 4
  eggs, 4 cups of flour, and 12 oz. of chocolate
  chips and produces 45 cookies.
• How many batches can be made from 1 dozen eggs,
  15 cups of flour, and 36 oz. of chocolate chips?
• How many cookies will be produced from 8 eggs, 4
  cups of flour, and 24 oz. of chocolate chips?

3 batches (eggs are limiting reactant)
Flour is limiting reactant, only 1 batch, so 45
cookies
16
Limiting Reactants

• H2 Cl2 ? 2HCl
• How many grams of hydrogen chloride can be
  produced from 0.490 g of hydrogen and 50.0 g of
  chlorine?
• Calculate mass-mass for BOTH reactants
• The one with the LEAST product is your limiting
  reactant (and thus your answer)

17
Limiting Reactants

• If 36.5 g HCl is reacted with 85.6 g of Ba(OH)2
  which is the limiting reactant? How much BaCl2
  will be produced?
• Barium hydroxide is the limiting reactant
• 104 g BaCl2 will be produced

18
Percent Yield

• Thus far, quantities represent maximum yield
  (100)
• Actual yield in lab is not often 100
• Side reactions
• Many reactions reversible
• Poor lab skills
• Expected (theoretical) yield
• calculated amount of product that can be obtained
• Actual yield
• Amount actually obtained

19
Percent Yield

• Percent yield
• Ratio of actual yield to theoretical yield
• Actual yield x 100 percent yield
• Expected yield
• Determine the percent yield for the reaction
  between 2.80 g Al(NO3)3 and excess NaOH if 0.966
  g Al(OH)3 is recovered. (your second product is
  NaNO3

20
Percent Yield

• A student places an iron nail with a mass of 2.32
  g into a flask of CuSO4. The nail reacts
  completely, leaving a quantity of copper metal in
  the bottom of the flask. The student finds the
  mass of the recovered copper to be 2.51 g. The
  equation for this reaction is Fe CuSO4 ?
  FeSO4 Cu
• What is the expected yield?
• What is the percent yield?

21
Percent Yield

• When octane (C8H18) is burned in oxygen, carbon
  dioxide and water are produced. If 320 g of
  octane are burned and 392 g of water are
  recovered, what is the percent yield of the
  experiment?
• First, write and balance the equation
• Then, answer the question

22
Percent Yield

• 4Al 3O2 ? 2Al2O3
• If 0.25 mol Al and 0.40 mol O2 are reacted, which
  is the limiting reactant?
• If a student reports a percent yield of 75.5,
  how much product (in grams) did she recover?

23
Part Two Homework

• Questions 1 2
• Paired Exercises 21, 23, 27 29
• Additional Exercises 31 41
• Be prepared for TEST on Thursday
• Last EXAM of the quarter
• Remember, EC due December 9th