The Mathematics of Chemical
Equations
Stoichiometry
Stoichiometry
• I. Stoichiometry- study of quantitative, or
measurable, relationships that exist in chemical
formulas and chemical reactions.
• II. Balanced Equations- The coefficients in a
balanced chemical reaction indicate the number of
moles of each substance participating in the
chemical reaction.
– Ex: 2H2 + O2  2H2O
– 2 moles of hydrogen + 1 mole of oxygen yields 2 moles
of water.
Mole-Mole problems
• Using a balanced equation you can determine
amounts of reactants or products.
– Ex: Nitrous oxide is produced by the decomposition of
ammonium nitrate.
• NH4(NO3)  N2O + 2H2O
• How many moles of N2O are produced from 2.25 moles of
NH4NO3?
Moles of given
Use molar ratio
to convert
Moles of unknown
Stoichiometry problems
• All stoichiometry problems are solved using
a balanced chemical equation
• By doing so, you verify the Law of
Conservation of Mass.
Mass-Mass Problems
• How many g of Cu(NO3)2 will be produced from 128 g of
AgNO3?
• Cu + 2 AgNO3  Cu(NO3)2 + 2Ag
Mass of given
Use molar
mass
Moles of given
Mass of unknown
Use molar
ratio
Use molar
mass
Moles of unknown
Mass-mole problems
• How many moles of CO2 are produced from 64.8 g CO2?
• 2C2H2 + 5O2  4CO2 + 2 H2O
convert
Mass of given
Moles of
given
Use molar
ratio
Moles of
unknown
Mole-mass problems
• What mass of H2O can be produced from 5.42
moles of glucose?
• C6H12O6 + 6 O2  4CO2 + 2 H2O
Moles of
given
Use molar
ratio
Moles of
unknown
Use molar
mass
Mass of
unknown
Mass-Volume problems
• Sodium azide is the compound found in air bags. In a
collision a sensor sets off a spark causing the NaN3 to
decompose explosively, producing solid sodium and
nitrogen gas: 2 NaN3  2Na + 3N2
• Assume an air bag contains 125 g of NaN3, what volume
of N2 is produced?
• (Flow chart on next slide)
Mass-volume problems
Mass of
given
Use molar
mass
Moles of
given
Volume of
Unknown gas
Use molar
volume
Use molar ratio
Moles of
unknown
Volume-volume problems
What volume of hydrogen gas is needed to react with 17.3 L
of nitrogen gas?
N2 + 3H2  2NH3
Volume of
given
Volume of
Unknown
Use molar
volume
Moles of
given
Use Molar
volume
Use molar ratio
Moles of unknown
Limiting Reactants
• The limiting reactant (LR) is completely used up
in the reaction.
• The other reactants will be left over, or “in
excess”
• The quantity (amount) of products formed in a rxn
are always determined by the quantity of the
limiting reactant
• To determine the LR you do mass-mass problems
Limiting Reactant
• Determine the LR:
• Cu + 2AgNO3  Cu(NO3)2 + 2Ag
Mass of
given
Moles of
given
Determine
LR
Moles of
unknown
Mass of
unknown
Percent Yield
• Ratio of the expected yield/actual yield
• Expected yield – amt of product that should
be produced according to the balanced
chemical equation.
• Actual yield- amt of product actually
produced in the reaction
Percent yield
• % yield = (actual yield/expected yield) x
100
• Reasons for less than 100% yield
–
–
–
–
1.
2.
3.
4.
Reactants don’t react
Side reactions
Loss of reactants/products
Operator error