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2/25/2014
1
STOICHIOMETRY
Stoikheion = element; metron = to measure
STOICHIOMETRY Chemists use balanced chemical equations to
calculate how much reactant is needed or how
much product is formed.
– provides the same kind of quantitative
information that a recipe does.
STOICHIOMETRY is…. • Calculation of the quantities of reactants
and products in a chemical reaction.
• The study of the quantitative relationships
between the amounts of reactants and
products in a chemical reaction.
• Based on the Law of Conservation of Mass
– MASS and the NUMBER OF ATOMS are
conserved in every chemical reaction.
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Interpreting Chemical
Equations • A balanced chemical equation can be
interpreted in terms of different quantities,
including numbers of atoms, molecules, or
moles; mass; and volume.
Interpreting Chemical
Equations
Number of Atoms
12.1
Interpreting Chemical
Equations
Number of Molecules
12.1
1 molecule N2 + 3 molecules H2 2 molecules NH3
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Interpreting Chemical
Equations
Number of Molecules
12.1
1 molecule N2 + 3 molecules H2 2 molecules NH3
Interpreting Chemical
Equations
Moles
12.1
1 molecule N2 + 3 molecules H2 2 molecules NH3
1 mole N2 + 3 moles H2 2 moles NH3
Interpreting Chemical
Equations
Mass
12.1
1 molecule N2 + 3 molecules H2 2 molecules NH3
1 mole N2 + 3 moles H2 2 moles NH3
28 g N2 + 3 x 2 g H2 2 x 17 g NH3
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Interpreting Chemical
Equations
Mass
12.1
1 molecule N2 + 3 molecules H2 2 molecules NH3
1 mole N2 + 3 moles H2 2 moles NH3
28 g N2 + 3 x 2 g H2 2 x 17 g NH3
34 g reactants 34 g products
Interpreting Chemical
Equations
Volume
12.1
1 molecule N2 + 3 molecules H2 2 molecules NH3
1 mole N2 + 3 moles H2 2 moles NH3
28 g N2 + 3 x 2 g H2 2 x 17 g NH3
34 g reactants 34 g products
Mole Ratios in Chemical Reactions
• The ratio between the coefficients of any two
compounds in a balanced chemical reaction.
EXAMPLE: 4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)
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Stoichiometry Vocabulary • Stoikheion = element; metron = to measure
• Study of the quantitative relationships in
chemical formulas & equations
• Review: a mole (mol) is a certain number of things
(atoms, ions or molecules) = to 6.02 x 1023
• Coefficients in the balanced equation tell you how
many moles of each reactant and product are needed
Stoichiometry Vocabulary
• Review: formula or molar mass is found by
adding the atomic masses on the periodic
table (round to the tenths place)
• Molar volume (measured at STP “standard
temperature & pressure”) of any gas is equal
to 22.4 L
Chapter 11 v. Chapter 12 In Chapter 11, we converted amounts of the
SAME substance to different units (moles,
particles, L or g).
In Chapter 12, we will be looking
at chemical equations and
calculating amounts of a
substance using a given amount
of a DIFFERENT substance.
Mole ratios are the
HEART of every
stoichiometry problem.
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Mole Map
Volume Volume
Given Wanted
Particles Moles Moles Particles
Given Given Wanted Wanted
Mass Mass
Given Wanted
Mole Map
Mass Moles Moles Mass
Given Given Wanted Wanted
Mole-Mole Conversions
Mole ratios are the HEART of every
stoichiometry problem.
Given (moles of the
known) # moles unknown
= calculated moles of unknown
# moles known
MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
The mole ratio you use depends on what
substance your given is (“known”) and which
substance you want to calculate (“unknown”)
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• Problem: If I have 2 moles of N2 and excess
H2, how many moles of NH3 can I make?
“KNOWN” (given):
“UNKNOWN”:
MOLE RATIO:
Mole-Mole Conversions - Example
1 N2 + 3 H2 2 NH3
2 moles N2
moles NH3
Mole-Mole Conversions - Example
N2 + 3 H2 2 NH3
Given (moles of the
known) # moles unknown
= calculated moles of unknown
# moles known
MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
2 moles N2 2 moles NH3 = 4 moles NH3
1 mole N2
• Problem: If I have 8 moles of O2 and excess
H2, how many moles of H2O can I make?
“KNOWN” (given):
“UNKNOWN”:
MOLE RATIO:
Mole-Mole Conversions - Example
2 H2 + 1 O2 2 H2O
8 moles O2
moles H2O
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Mole-Mole Conversions - Example
2 H2 + O2 2 H2O
Given (moles of the
known) # moles unknown
= calculated moles of unknown
# moles known
MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
8 moles O2 2 moles H2O = 16 moles H2O
1 mole O2
Mole-Mole Calculations – You try…
4 Al + 3 O2 2 Al2O3
1. KNOWN: UNKNOWN:
12 moles Al2O3 moles O2
2. Mole ratio:
3. Calculate unknown:
32
2
OAl moles 2
O moles 3
known moles
unknown moles
2O moles 18 32
232
OAl moles 2
O moles 3OAl moles 12
•If I have 12 moles of Al2O3 how many moles of O2 do I need?
Mole Map
Mass Moles Moles Mass
Given Given Wanted Wanted
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Mole-Mass Conversions
Conversion if you know the number
of moles of a reactant or product in
a reaction and you want to calculate
the mass of another product or
reactant.
Mole-Mass Calculations - Example
Problem:
• If I have 2 moles of N2 and excess H2, how many grams
of NH3 can I make?
N2 + 3 H2 2 NH3
Given (moles of
the known) # moles unknown g unknown = calculated
mass (g) of
unknown # moles known 1 mole unknown
MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
MOLAR MASS OF UNKNOWN
2 moles N2 2 moles NH3 17.0 g NH3 = 68.0 g NH3
1 mole N2 1 mole NH3
Mole-Mass Calculations - Example
Problem:
• If I have 8 moles of O2 and excess H2, how many grams
of H2O can I make?
2 H2 + O2 2 H2O
Given (moles of
the known) # moles unknown g unknown = calculated
mass (g) of
unknown # moles known 1 mole unknown
MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
MOLAR MASS OF UNKNOWN
8 moles O2 2 moles H2O 18.0 g H2O
= 288.0 g H2O 1 mole O2
1 mole H2O
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Mass-Mole Calculation - Example
Problem:
• If I have 8 grams of O2 and excess H2, how many moles
of H2O can I make?
2 H2 + O2 2 H2O
Given (grams of
the known) 1 mole known # moles unknown = calculated
moles of
unknown grams known # moles known
MOLAR MASS OF KNOWN MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
8 g O2 1 mole O2 2 moles H2O = 0.5 mole
H2O 32.0 g O2
1 mole O2
Mole Map
Mass Moles Moles Mass
Given Given Wanted Wanted
Mass-Mass Conversions Conversion if you know the mass of a reactant or
product in a reaction and you want to calculate the
mass of another product or reactant.
Given
(grams
known) 1 mole known # moles unknown grams unknown = calculated
mass (g) of
unknown grams known # moles known 1 mole unknown
MOLAR MASS OF
KNOWN
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
MOLAR MASS OF
UNKNOWN
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11
Mass-Mass Calculations - Example
Problem:
• If I have 2.0 grams of N2 and excess H2, how many grams
of NH3 can I make?
N2 + 3 H2 2 NH3
Given
(grams
known) 1 mole known # moles unknown grams unknown
= calculated
mass (g) of
unknown grams known # moles known 1 mole unknown
MOLAR MASS OF
KNOWN
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
MOLAR MASS OF
UNKNOWN
2.0 g N2 1 mole N2 2 moles NH3 17.0 grams NH3
= 2.4 g NH3
28.0 grams N2 1 moles N2 1 mole NH3
Mass-Mass Calculations - Example
Problem:
• If I have 37.0 grams of H2, how many grams of O2 will
react?
2 H2 + O2 2 H2O
Given
(grams
known) 1 mole known # moles unknown grams unknown
= calculated
mass (g) of
unknown grams known # moles known 1 mole unknown
MOLAR MASS OF
KNOWN
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
MOLAR MASS OF
UNKNOWN
37.0 g H2 1 mole H2 1 moles O2 32.0 grams O2
= 296 g O2
2.0 grams H2 2 moles H2 1 mole O2
Mass-Mass Calculations - Example
Problem:
• If I have 37.0 grams of H2, how many grams of O2 will
react?
2 H2 + O2 2 H2O
Given
(grams
known) 1 mole known # moles unknown grams unknown
= calculated
mass (g) of
unknown grams known # moles known 1 mole unknown
MOLAR MASS OF
KNOWN
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
MOLAR MASS OF
UNKNOWN
37.0 g H2 1 mole H2 1 moles O2 32.0 grams O2
= 296 g O2
2.0 grams H2 2 moles H2 1 mole O2
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Mole Map
Mass Moles Moles Mass
Given Given Wanted Wanted
Mixed Stoichiometry Calculations
Problem:
• If I have 37.0 grams of H2, how many moles of O2 will
react?
2 H2 + O2 2 H2O
Given
(grams
known) 1 mole known # moles unknown = calculated
moles of
unknown grams known # moles known
MOLAR MASS OF
KNOWN
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
Mass Moles Moles Mass
Given Given Wanted Wanted
Mixed Stoichiometry Calculations
Problem:
• If I have 2.0 moles of H2, how many grams of O2 will
react?
2 H2 + O2 2 H2O
Mass Moles Moles Mass
Given Given Wanted Wanted
Given (moles
known) # moles unknown
grams
unknown = calculated
mass (g) of
unknown # moles known
1 mole
unknown
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
MOLAR MASS OF
UNKNOWN
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13
Mixed Stoichiometry Calculations
Problem:
• If I have 2.0 moles of H2, how many moles of O2 will
react?
2 H2 + O2 2 H2O
Mass Moles Moles Mass
Given Given Wanted Wanted
Given (moles
known) # moles unknown = calculated
moles of
unknown # moles known
MOLE RATIO USING
COEFFICIENTS FROM CHEM.
RXN.
Mixed Stoichiometry Calculations
Problem:
• If I have 37.0 grams of H2, how many grams of O2 will
react?
2 H2 + O2 2 H2O
Given
(grams
known) 1 mole known # moles unknown grams unknown
= calculated
mass (g) of
unknown grams known # moles known 1 mole unknown
MOLAR MASS OF
KNOWN
MOLE RATIO USING
COEFFICIENTS FROM
CHEM. RXN.
MOLAR MASS OF
UNKNOWN
Mass Moles Moles Mass
Given Given Wanted Wanted