AP Chemistry
Stoichiometry
HW: 1 5 11 13 17 19 25 35 39 49
57 59 63 73 77 89 101
3.1 – Chemical Equations
+ = Reacts with
-> = Yields / Produces
Reactants = Substances on the left side of
the arrow
Products = Substances on the right side of
the arrow
Balancing Reactions
For now the old method is OK. We’ll learn
more about balancing more complicated
(redox) reactions later
1. Write correct formulas
2. Balance elements / ions that only appear
once on each side
3. Balance others
4. Balance O and H
5. Check
3.1 – Balancing Equations
Most can be done by inspection for now!
Ex – methane + oxygen -> carbon dioxide +
water
Reaction Example
CH4 (g) + 2 O2 (g)

CO2 (g) + 2 H2O (g)
3.1 – Balancing Equations
Ex – sodium + water -> sodium hydroxide +
hydrogen gas
Reaction
Types
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Combination Reactions (AKA
Synthesis)
• In this type of
reaction two
or more
substances
react to form
one product.
• Examples:
– 2 Mg (s) + O2 (g)  2 MgO (s)
– N2 (g) + 3 H2 (g)  2 NH3 (g)
– C3H6 (g) + Br2 (l)  C3H6Br2 (l)
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Decomposition Reactions
• In a decomposition
one substance breaks
down into two or more
substances.
• Examples:
– CaCO3 (s)  CaO (s) + CO2 (g)
– 2 KClO3 (s)  2 KCl (s) + 3O2 (g)
– 2 NaN3 (s)  2 Na (s) + 3 N2 (g)
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Combustion Reactions
• These are generally
rapid reactions that
produce a flame.
• Most often involve
hydrocarbons
reacting with oxygen
in the air.
• Examples:
– CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
– C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (g)
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AP Question 4
-Changed for 2008
-Now want BALANCED NET-IONIC
-Ask you an additional question
-No “choice” in the questions
**See additional packet for practice
Be sure to know:
-Single displacement (metal and nonmetal)
-Double displacement
-Combustion
-Acid / Base (we’ll cover more later)
AP Question 4
3.3 – Formula Weights
Atomic mass – Mass of an atom in amu (atomic
mass unit)
1 amu = 1/12 of the mass of a Carbon-12 atom
Mole = Amount of a substance based on
particles
Avogadro’s Number = # of particles in 1 mole;
Based on 12 grams of Carbon–12 = 6.02 x
1023
Molar Mass = Mass needed to obtain Avogadro’s
Number of particles for any substance.
3.3 – Atomic Weight / Formula
Weight / Molecular Weight
• A formula weight is the sum of the
atomic weights for the atoms in a
chemical formula.
• So, the formula weight of calcium
chloride, CaCl2, would be
Ca: 1(40.1 amu)
+ Cl: 2(35.5 amu)
111.1 amu
• Formula weights are generally reported
for ionic compounds.
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Molecular Weight (MW)
• A molecular weight is the sum of the
atomic weights of the atoms in a
molecule.
• For the molecule ethane, C2H6, the
molecular weight would be
C: 2(12.0 amu)
+ H: 6(1.0 amu)
30.0 amu
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Percent Composition
One can find the percentage of the
mass of a compound that comes from
each of the elements in the compound
by using this equation:
(number of atoms)(atomic weight)
x 100
% element =
(FW of the compound)
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3.4 - Avogadro’s Number
• 6.02 x 1023 particles
• 1 mole of 12C has a mass of 12 g.
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Molar Mass
• By definition, a molar mass is the mass of
1 mol of a substance (i.e., g/mol).
– The molar mass of an element is the mass
number for the element that we find on the
periodic table.
– The formula weight (in amu’s) will be the
same number as the molar mass (in g/mol).
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Using Moles
Moles provide a bridge from the molecular
scale to the real-world scale.
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Practice Problems
Mole Relationships
• One mole of atoms, ions, or molecules contains
Avogadro’s number of those particles.
• One mole of molecules or formula units contains
Avogadro’s number times the number of atoms or
ions of each element in the compound.
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Practice Problems
3.5 - Calculating Empirical
Formulas
One can calculate the empirical formula from
the percent composition.
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Combustion Analysis
• Compounds containing C, H and O are routinely
analyzed through combustion in a chamber like this.
– C is determined from the mass of CO2 produced.
– H is determined from the mass of H2O produced.
– O is determined by difference after the C and H have been
determined.
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Elemental Analyses
Compounds
containing other
elements are
analyzed using
methods analogous
to those used for C,
H and O.
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Calculating Empirical Formulas
The compound para-aminobenzoic acid (you may have seen it listed as
PABA on your bottle of sunscreen) is composed of carbon (61.31%),
hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the
empirical formula of PABA.
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Calculating Empirical Formulas
C7H7NO2
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3.5 – Empirical Formulas from
Lab Data
Determining Molecular Formula
-Need to know approximate Molar Mass
-Need to know Empirical Formula
-Molecular Formula is a multiple of the
Empirical Formula
Practice Problem
Caproic acid, which is responsible for the foul odor of
dirty socks, is composed of C, H and O atoms.
Combustion of a 0.225 g sample of this compound
produces 0.5112 g carbon dioxide and 0.209 g water.
Water is the empirical formula of caproic acid? If the
molar mass of the acid is 116 g/mol, what is the
molecular formula?
3.6 – Quantitative Information
from Balanced Equations
-Stoichiometry – Coefficients (numbers in
front of substances in a balanced
equation) provide relative numbers of
moles in the reaction
Examples:
Stoichiometric Calculations
Starting with the
mass of Substance
A you can use the
ratio of the
coefficients of A and
B to calculate the
mass of Substance
B formed (if it’s a
product) or used (if
it’s a reactant).
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Stoichiometric Calculations
C6H12O6 + 6 O2  6 CO2 + 6 H2O
Starting with 1.00 g of C6H12O6…
calculate the mass of water that would be
produced.
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Limiting Reactants
• You can make cookies
until you run out of one
of the ingredients.
• Once this family runs
out of sugar, they will
stop making cookies
(at least any cookies
you would want to eat).
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How Many Cookies Can I Make?
• In this example the
sugar would be the
limiting reactant,
because it will limit the
amount of cookies you
can make.
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Limiting Reactants
• The limiting reactant is the reactant present in
the smallest stoichiometric amount.
– In other words, it’s the reactant you’ll run out of first (in
this case, the H2).
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Limiting Reactants
In the example below, the O2 would be the
excess reagent.
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Limiting Reactants
• Best method is to do separate
stoichiometry calculations for each
reactant.
• Then compare the answers.
• The smallest answer is the amount of
product that will be produced.
Examples:
Theoretical Yield
• The theoretical yield is the maximum
amount of product that can be made.
– In other words it’s the amount of product
possible as calculated through the
stoichiometry problem.
• This is different from the actual yield,
which is the amount one actually produces
and measures.
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Percent Yield
One finds the percent yield by comparing
the amount actually obtained (actual
yield) to the amount it was possible to
make (theoretical yield).
Actual Yield
Percent Yield =
Theoretical Yield
x 100
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