CHEMISTRY
The Central Science
9th Edition
Chapter 3
Stoichiometry: Calculations with
Chemical Formulas and Equations
David P. White
Prentice Hall © 2003
Chapter 3
Chemical Equations
• Lavoisier: mass is conserved in
a chemical reaction.
• Chemical equations:
descriptions of chemical
reactions.
• Two parts to an equation:
reactants and products:
2H2 + O2  2H2O
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Chapter 3
Chemical Equations
• The chemical equation for the formation of water can be
visualized as two hydrogen molecules reacting with one
oxygen molecule to form two water molecules:
2H2 + O2  2H2O
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Chapter 3
Chemical Equations
2Na + 2H2O  2NaOH + H2
2K + 2H2O  2KOH + H2
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Chapter 3
Chemical Equations
• Stoichiometric coefficients: numbers in front of the
chemical formulas; give ratio of reactants and products.
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Chapter 3
Chemical Equations
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Chapter 3
Chemical Equations
• Law of conservation of mass: matter cannot be lost in any
chemical reactions.
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Chapter 3
Some Simple Patterns of
Chemical Reactivity
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•
•
•
Combination and Decomposition
Reactions
Combination reactions have fewer products than
reactants:
2Mg(s) + O2(g)  2MgO(s)
The Mg has combined with O2 to form MgO.
Decomposition reactions have fewer reactants than
products:
2NaN3(s)  2Na(s) + 3N2(g)
(the reaction that occurs in an air bag)
The NaN3 has decomposed into Na and N2 gas.
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Chapter 3
Some Simple Patterns of
Chemical Reactivity
Combination and Decomposition
Reactions
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Chapter 3
Some Simple Patterns of
Chemical Reactivity
Combination and Decomposition
Reactions
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Chapter 3
Some Simple Patterns of
Chemical Reactivity
Combustion in Air
Combustion is the burning of a
substance in oxygen from air:
C3H8(g) + 5O2(g)  3CO2(g) +
4H2O(l)
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Chapter 3
Formula Weights
Formula and Molecular Weights
• Formula weights (FW): sum of AW for atoms in formula.
FW (H2SO4) = 2AW(H) + AW(S) + 4AW(O)
= 2(1.0 amu) + (32.0 amu) + 4(16.0)
= 98.0 amu
• Molecular weight (MW) is the weight of the molecular
formula.
MW(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu)
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Chapter 3
Formula Weights
Percentage Composition from
Formulas
• Percent composition is the atomic weight for each
element divided by the formula weight of the compound
multiplied by 100:
% Element 
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Atoms of Element AW 
FW of Compound
Chapter 3
 100
The Mole
Mole: convenient measure chemical quantities.
• 1 mole of something = 6.0221367  1023 of that thing.
• Experimentally, 1 mole of 12C has a mass of 12 g.
Molar Mass
• Molar mass: mass in grams of 1 mole of substance (units
g/mol, g.mol-1).
• Mass of 1 mole of 12C = 12 g.
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Chapter 3
The Mole
The Mole
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Chapter 3
The Mole
This photograph shows one
mole of solid (NaCl), liquid
(H2O), and gas (N2).
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Chapter 3
The Mole
Interconverting Masses, Moles, and
Number of Particles
• Molar mass: sum of the molar masses of the atoms:
molar mass of N2 = 2  (molar mass of N).
• Molar masses for elements are found on the periodic
table.
• Formula weights are numerically equal to the molar
mass.
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Chapter 3
Empirical Formulas from
Analyses
• Start with mass % of elements (i.e. empirical data) and
calculate a formula, or
• Start with the formula and calculate the mass % elements.
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Chapter 3
Empirical Formulas from
Analyses
Molecular Formula from Empirical
Formula
• Once we know the empirical formula, we need the MW
to find the molecular formula.
• Subscripts in the molecular formula are always wholenumber multiples of subscripts in the empirical formula
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Chapter 3
Empirical Formulas from
Analyses
Combustion Analysis
• Empirical formulas are determined by combustion
analysis:
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Chapter 3
Quantitative Information
from Balanced Equations
• Balanced chemical equation gives number of molecules
that react to form products.
• Interpretation: ratio of number of moles of reactant
required to give the ratio of number of moles of product.
• These ratios are called stoichiometric ratios.
NB: Stoichiometric ratios are ideal proportions
• Real ratios of reactants and products in the laboratory
need to be measured (in grams and converted to moles).
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Chapter 3
Limiting Reactants
• If the reactants are not present in stoichiometric amounts,
at end of reaction some reactants are still present (in
excess).
• Limiting Reactant: one reactant that is consumed
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Chapter 3
Limiting Reactants
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Chapter 3
Limiting Reactants
Theoretical Yields
• The amount of product predicted from stoichiometry
taking into account limiting reagents is called the
theoretical yield.
• The percent yield relates the actual yield (amount of
material recovered in the laboratory) to the theoretical
yield:
Actual yield
% Yield 
 100
Theoretica l yield
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Chapter 3
End of Chapter 3:
Stoichiometry: Calculations with
Chemical Formulas and
Equations
Prentice Hall © 2003
Chapter 3