Chemical Stoichiometry

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Chapter 3

• Chemical Stoichiometry

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3.1 Atomic Mass/Weight
The average mass of a sulfur atom is 32.06 amu
S
The average mass of a sodium atom is 22.99 amu
Na
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3.2 Molecular Mass/Weight Formula Mass/Weight

• The formula of an Acetylsalicylic Acid (Aspirin)
  molecule is C9H8O4
• Acetylsalicylic Acid (Aspirin) has molecular mass
  of 180.159 amu

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3.2 Molecular Mass/Weight Formula Mass/Weight

• Aluminum sulfate is an ionic compound with the
  formula Al2(SO4)3
• Aluminum sulfate has a formula mass of 342.153
  amu.

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3.3 Isotopes

• Isotopes are atoms of an element that differ only
  in the number of neutrons in the nucleus of the
  atom.
• Chlorine has two isotopes
• chlorine 35 35Cl
• chlorine 37 37Cl

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3.3 Isotopes

• In any random sample of chlorine about 1 in 4
  atoms is chlorine 37.

The atomic mass of chlorine is therefore about
35.5 amu.
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3.3 Isotopes

• How do atoms of chlorine 35 and chlorine 37
  differ?

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3.4 Moles of Atoms and Avogadros Number

• What is a mole of atoms?

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3.4 Moles of Atoms and Avogadros Number

• What is a mole of atoms?

One mole of magnesium atoms contains 6.02 x 1023
Mg atoms.
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3.4 Moles of Atoms and Avogadros Number

• What is the mass of one mole of Mg atoms?

• The atomic mass of Mg is 24.305 amu.
• The molar mass of Mg is 24.305 grams.

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3.5 Moles of Molecules

• What is a mole of molecules?

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3.5 Moles of Molecules

• What is a mole of molecules?
• One mole of CHCl3 (chloroform) contains 6.02 x
  1023 molecules of CHCl3.

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3.5 Moles of Molecules

• What is a mole of molecules?
• One mole of CHCl3 (chloroform) contains 6.02 x
  1023 molecules of CHCl3.
• What else does it contain?

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3.5 Moles of Molecules

• How many H atoms are in one mole of CHCl3?

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3.5 Moles of Molecules

• How many H atoms are in one mole of CHCl3?
• 6.02 x 1023 H atoms

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3.5 Moles of Molecules

• How many Cl atoms are in one mole of CHCl3?

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3.5 Moles of Molecules

• How many Cl atoms are in one mole of CHCl3?
• 3(6.02 x 1023) Cl atoms

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3.5 Moles of Molecules

• What is the molar mass of CHCl3?

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3.5 Moles of Molecules

• What is the molar mass of CHCl3?
• 119.377g

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Key Concept

• You have one mole of ascorbic acid (Vitamin C)
  C6H8O6. What else do you have?
• 6.02 x 1023 molecules of C6H8O6
• 176.13 grams of C6H8O6
• 6 moles C or 6(6.02 x 1023) C atoms
• 8 moles H or 8(6.02 x 1023) H atoms
• 6 moles O or 6(6.02 x 1023) O atoms

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Key Concept

• If you have a certain number of moles of any
  compound you can always find the moles of each
  element present?
• How many mol C are in 0.80 mol of CO2?
• How many mol O are in 0.80 mol of CO2?
• How many mol H are in 1.2 mol of C12H22O11?
• How many mol S are in 3.8 mol of Cr(SO4)3?
• How many mol O are in 3.8 mol of Cr(SO4)3?

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3.6 Percentage Composition or Percent by Mass

• What is the percent by mass of hydrogen in water?

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3.6 Percentage Composition or Percent by Mass
A glass of water contains 126g of water. How
many grams of hydrogen are present?
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3.7 Derivation of Formulas

• To calculate a formula of a substance we often
  use a mole ratio.

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3.7 Derivation of Formulas

• To calculate a formula of a substance we often
  use a mole ratio.
• Analysis of a gas shows that it is composed of
  0.090 mol carbon and 0.36 mol hydrogen. What is
  the empirical formula gas?

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3.7 Derivation of Formulas
What is the empirical formula of a compound that
is 27.29 carbon and 72.71 oxygen.
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3.7 Derivation of Formulas
A sample of hematite contains 34.97g of iron and
15.03g of oxygen. What is the empirical formula
of hematite?
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Determine the empirical and molecular formulas
for a compound with the following elemental
composition 40.00 C, 6.72 H, 53.29 O. The
molar mass of the compound is 180. g/mol.
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Determine the empirical and molecular formulas
for a compound with the following elemental
composition 40.00 C, 6.72 H, 53.29 O. The
molar mass of the compound is 180. g/mol.
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Determine the empirical and molecular formulas
for a compound with the following elemental
composition 40.00 C, 6.72 H, 53.29 O. The
molar mass of the compound is 180. g/mol.
CH2O
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Determine the empirical and molecular formulas
for a compound with the following elemental
composition 40.00 C, 6.72 H, 53.29 O. The
molar mass of the compound is 180. g/mol.
CH2O

• 12.01 2(1.008) 1 (16.00) 30.03
• 180./30.03 6
• 6(CH2O) C6H12O6

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3.7 Derivation of Formulas
3.22g of a compound decomposes when heated into
1.96g of KCl and oxygen. What is the empirical
formula of the compound?
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3.8 Solutions

• Solute The substance that dissolves (the minor
  component of a solution).

KMnO4
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3.8 Solutions

• Solvent The substance in which the solute
  dissolves (the major component of a solution).

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Solution A homogeneous mixture of the solute
and solvent.
KMnO4 solution
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3.8 Solutions
Dilute Concentrated
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Concentration of a solution

• Molarity(M) is the moles of solute per liter of
  solution.

moles of solute
Molarity(M)
Liters of solution
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• The next slide lasts between 5 and 6 minutes and
  can be skipped if needed.

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Preparation of Solutions
28.3 grams of nickel(II) chloride
Add water to make the desired volume (500.0 ml)
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Molarity

• Calculate the molarity of an aqueous nickel (II)
  chloride solution containing 28.3 grams of nickel
  (II) chloride in 500.0 mL of solution.

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Molarity

• The maximum solubility of lead (II) chromate,
  PbCrO4, is 4.3 x 10-5 g/L. What is the molarity
  of a saturated solution of PbCrO4?

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Molarity

• How many moles of sulfuric acid, H2SO4, are
  contained in 0.80L of a 0.050M solution of
  sulfuric acid?
• How many grams of H2SO4 would be needed to make
  this solution?

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Dilution

• If we take a more concentrated solution we can
  dilute it to a lower concentration by adding
  water to it.
• We determine the concentration (molarity) of the
  diluted solution by using the following formula.
• M1V1 M2V2

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Dilution

• Calculate the concentration of the resulting
  solution when enough water is added to 250.0mL of
  0.60M NaOH to make 300.0mL of solution?

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Dilution

• How much water would need to be added to 125mL of
  a 1.50M solution of HCl to dilute the solution to
  a concentration of 0.570M?

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Density

• An understanding of density is often necessary in
  solving various problems.
• Density is a ratio of mass to volume and
  therefore can be used in the conversion of mass
  to volume or volume to mass.

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Mass Percent

• An understanding of mass percent is often
  necessary in solving various problems.
• Mass percent indicates the percentage of a
  particular substance in a mixture.

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Aniline, C6H5NH2, a key ingredient in the
preparation of dyes for fabrics, is produced by
the reaction of C6H5Cl with a solution containing
28.2 NH3 by mass. If the density of the NH3
solution is 0.899g/cm3. What mass of NH3 is
needed to prepare 125mL of the ammonia solution?
aniline
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Aniline, C6H5NH2, a key ingredient in the
preparation of dyes for fabrics, is produced by
the reaction of C6H5Cl with a solution containing
28.2 NH3 by mass. If the density of the NH3
solution is 0.899g/cm3. What mass of NH3 is
needed to prepare 125mL of the ammonia solution?
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Stoichiometry Calculations based on balanced
equations.

• We use the balanced equation to determine a mole
  ratio.
• Then we use Moles to Move.

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Stoichiometry Calculations based on balanced
equations.

• How many moles of Al2I6 are produced from 0.40
  mol Al?

?

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Stoichiometry Calculations based on balanced
equations.

• Calculate the moles of oxygen produced by the
  thermal decomposition of 100.0g of potassium
  chlorate.

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Stoichiometry Calculations based on balanced
equations.

• Calculate the moles of oxygen produced by the
  thermal decomposition of 100.0g of potassium
  chlorate.
• Write the equation for this reaction.

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• Calculate the moles of oxygen produced by the
  thermal decomposition of 100.0g of potassium
  chlorate.
• Write the equation for this reaction.
• 2KClO3 ? 2KCl 3O2

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• Calculate the moles of oxygen produced by the
  thermal decomposition of 100.0g of potassium
  chlorate.
• 2KClO3 ? 2KCl 3O2

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• The antacid milk of magnesia contains Mg(OH)2.
  What mass of NaOH would be needed to produce 16g
  of Mg(OH)2? MgCl2 2NaOH ? Mg(OH)2 2NaCl

.
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• The antacid milk of magnesia contains Mg(OH)2.
  What mass of NaOH would be needed to produce 16g
  of Mg(OH)2?
• MgCl2 2NaOH ? Mg(OH)2 2NaCl

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What mass of oxygen is required to burn 702g of
octane?
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What mass of oxygen is required to burn 702g of
octane? Write the balanced equation for this
reaction.
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What mass of oxygen is required to burn 702g of
octane? 2C8H18 25O2 ? 16CO2 18H2O
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• What volume of 0.750M HCl can be made from 25.0g
  of NaCl?
• NaCl H2SO4 ? HCl NaHSO4

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• What volume of 0.2089M KI solution reacts with
  43.88mL of 0.3842M Cu(NO3)2?
• 2Cu(NO3)2 4KI ? 2CuI I2 4KNO3

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Percentage Yield
actual yield
Percent Yield
X 100
theoretical yield
Percentage yield can be a part of stoichiometry
problems
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Percentage Yield

• 1.274g of CuSO4 produces 0.392g of Cu. What is
  the percentage yield?
• CuSO4 Zn ? Cu ZnSO4

?
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Percentage Yield

• 1.274g of CuSO4 produces 0.392g of Cu. What is
  the percentage yield?
• CuSO4 Zn ? Cu ZnSO4

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Limiting Reagents

• A limiting reagent (reactant) is the reactant
  that is used up in a chemical reaction.
• All other reactants are said to be in excess.

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Limiting Reagent
NaHCO3 HC2H3O2 ? NaC2H3O2 H2CO3

• 195g NaHCO3 and 152ml 3.0M HC2H3O2

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Limiting Reagent
NaHCO3 HC2H3O2 ? NaC2H3O2 H2O CO2

• 195g NaHCO3 and 152ml 3.0M HC2H3O2

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Limiting Reagent
NaHCO3 HC2H3O2 ? NaC2H3O2 H2O CO2

• 195g NaHCO3 and 152ml 3.0M HC2H3O2
• Which reactant is limiting?

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Limiting Reagent
NaHCO3 HC2H3O2 ? NaC2H3O2 H2O CO2

• 195g NaHCO3 and 152ml 3.0M HC2H3O2
• Which reactant is limiting?
• How many grams of CO2 are produced?

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Limiting Reagent
NaHCO3 HC2H3O2 ? NaC2H3O2 H2O CO2

• 195g NaHCO3 and 152ml 3.0M HC2H3O2
• Which reactant is limiting?
• How many grams of CO2 are produced?
• How many grams of NaHCO3 remain after the
  reaction?

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Limiting Reagent
NaHCO3 HC2H3O2 ? NaC2H3O2 H2O CO2

• 195g NaHCO3 and 152ml 3.0M HC2H3O2
• How many grams of NaHCO3 remain after the
  reaction?

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A mixture of 5.0g H2 and 10.0g of O2 is ignited
forming water. How much water will the reaction
produce?
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Titration

• Titration is a common laboratory method of
  quantitative chemical analysis that is used to
  determine the unknown concentration of a
  reactant.
• Because volume measurements play a key role in
  titration, it is also known as volumetric
  analysis.
• A reagent, called the titrant, is placed in a
  calibrated buret and reacted with another
  solution.
• One of the solutions is of known concentration (a
  standard solution). It is used to react with a
  solution whose concentration is not known.
• An indicator is used to identify the endpoint of
  the reaction. The point at which the reaction is
  complete. The endpoint is often indicated by a
  permanent change in color due to the indicator.

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Titration
Before endpoint
Endpoint
Overshoot
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Titration (Acid Base)
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0.5105g NaOH is dissolved in water and titrated
to an endpoint with 48.47 mL of HCl. What is the
concentration of HCl? NaOH HCl ? NaCl H2O
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A solution of 20.00mL of oxalic acid was titrated
to an endpoint with 23.23mL of 0.09113M potassium
permanganate. What is the concentration of the
oxalic acid solution. 5H2C2O4 2KMnO4 3H2SO4 ?
2MnSO4 K2SO4 10CO2 8H2O
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How many grams of a sample containing 75.0
calcium hydroxide by mass is required to react
with the acetic acid in 25.0mL of a solution
having a density of 1.065 g/mL and containing
58.0 acetic acid by mass? Ca(OH)2
2HC2H3O2 ? Ca(C2H3O2)2 2H2O