Chapter 3
Chemical
Stoichiometry
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
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
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.
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
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.
3.3: Isotopes
• How do atoms of chlorine – 35 and
chlorine – 37 differ?
3.4: Moles of Atoms and
Avogadro’s Number
• What is a mole of atoms?
3.4: Moles of Atoms and
Avogadro’s Number
• What is a mole of atoms?
One mole of magnesium atoms contains
6.02 x 1023 Mg atoms.
3.4: Moles of Atoms and
Avogadro’s 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.
3.5: Moles of Molecules
• What is a mole of molecules?
3.5: Moles of Molecules
• What is a mole of molecules?
• One mole of CHCl3 (chloroform)
contains 6.02 x 1023 molecules of
CHCl3.
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?
3.5: Moles of Molecules
• How many H atoms
are in one mole of
CHCl3?
3.5: Moles of Molecules
• How many H atoms
are in one mole of
CHCl3?
6.02 x 1023 H atoms
3.5: Moles of Molecules
• How many Cl atoms
are in one mole of
CHCl3?
3.5: Moles of Molecules
• How many Cl atoms
are in one mole of
CHCl3?
3(6.02 x 1023) Cl atoms
3.5: Moles of Molecules
• What is the molar
mass of CHCl3?
3.5: Moles of Molecules
• What is the molar
mass of CHCl3?
119.377g
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
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?
3.6: Percentage Composition or
Percent by Mass
•What is the percent by mass of
hydrogen in water?
3.6: Percentage Composition or
Percent by Mass
A glass of water contains 126g of
water. How many grams of
hydrogen are present?
3.7: Derivation of Formulas
•To calculate a formula of a substance we often use a
mole ratio.
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?
3.7: Derivation of Formulas
What is the empirical formula of a compound that is
27.29% carbon and 72.71% oxygen.
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?
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.
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.
1 mol C
40.00 g C x
 3.331 mol C
12.01 g C
1 mol H
6.72 g H x
 6.667 mol H
1.008 g H
1 mol O
53.29 g O x
 3.331 mol O
16.00 g O
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.
3.331 mol C
 1.000 C  1 C
3.331 mol
6.667 mol H
 2.001 H  2 H
3.331 mol
3.331 mol O
 1.000 O  1 O
3.331 mol
CH2O
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
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?
3.8: Solutions
• Solute: The substance that dissolves (the
minor component of a solution).
KMnO4
3.8: Solutions
• Solvent: The substance in
which the solute dissolves
(the major component of a
solution).
Solution: A homogeneous mixture
of the solute and solvent.
KMnO4 solution
3.8: Solutions
Dilute
Concentrated
Concentration of a solution
Molarity(M) is the moles of solute per liter of
solution.
moles of solute
Molarity(M) =
Liters of solution
Preparation of Solutions
28.3 grams of
Add water to make the desired volume (500.0 ml)
nickel(II) chloride
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.
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?
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?
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
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?
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?
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.
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.
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
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?
Stoichiometry: Calculations based
on balanced equations.
• We use the balanced equation to determine a
mole ratio.
• Then we use “Moles to Move”.
Stoichiometry: Calculations based
on balanced equations.
How many moles of Al2I6 are produced from 0.40 mol Al?
+
→
Stoichiometry: Calculations based
on balanced equations.
Calculate the moles of oxygen produced by the thermal
decomposition of 100.0g of potassium chlorate.
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.
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
Calculate the moles of oxygen produced by the thermal
decomposition of 100.0g of potassium chlorate.
2KClO3 → 2KCl + 3O2
• 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
.
• 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
What mass of oxygen is required to burn 702g of octane?
What mass of oxygen is required to burn 702g of octane?
Write the balanced equation for this reaction.
What mass of oxygen is required to burn 702g of octane?
2C8H18 + 25O2 → 16CO2 + 18H2O
•What volume of 0.750M HCl can be made from 25.0g
of NaCl?
NaCl + H2SO4 → HCl + NaHSO4
•What volume of 0.2089M KI solution reacts with
43.88mL of 0.3842M Cu(NO3)2?
2Cu(NO3)2 + 4KI → 2CuI + I2 + 4KNO3
Percentage Yield
Percent Yield =
actual yield X 100
theoretical yield
Percentage yield can be a part of stoichiometry problems
Percentage Yield
• 1.274g of CuSO4 produces 0.392g of Cu. What
is the percentage yield?
CuSO4 + Zn → Cu + ZnSO4
→
Percentage Yield
• 1.274g of CuSO4 produces 0.392g of Cu. What
is the percentage yield?
CuSO4 + Zn → Cu + ZnSO4
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.
Limiting Reagent
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2CO3
• 195g NaHCO3 and 152ml 3.0M HC2H3O2
Limiting Reagent
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2
• 195g NaHCO3 and 152ml 3.0M HC2H3O2
Limiting Reagent
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2
• 195g NaHCO3 and 152ml 3.0M HC2H3O2
• Which reactant is limiting?
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?
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?
Limiting Reagent
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2
• 195g NaHCO3 and 152ml 3.0M HC2H3O2
• How many grams of NaHCO3 remain after the reaction?
A mixture of 5.0g H2 and 10.0g of O2 is ignited forming
water. How much water will the reaction produce?
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.
Titration
Before endpoint
Endpoint
Overshoot
Titration (Acid – Base)
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
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
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