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Stoichiometry
Chemistry – Chapter 12
What is Stoichiometry?
 Chemical reactions, such as the combustion of a candle, stop
when one of the reactants is used up
 Stoichiometry – study of quantitative relationships between
amounts of reactants used and products formed by a
chemical reaction
 Stoichiometry and balanced chemical equations:
Example:
4Fe + 3O2
2Fe2O3
 4 atoms of Fe react w/ 3 molecules of O to produce 2 formula
units of Fe2O3
 4 mol of Fe react w/ 3 mol of O to produce 2 mol of Fe2O3
 Mole ratio – ratio between the numbers of moles of any 2
substances in a balanced chemical equation
Example:
2Al + 3Br2
2AlBr3
(See p. 356 for mole ratios)
Stoichiometric Calculations
 Steps in Stoichiometric Calculations:
1.
2.
3.
4.
Write a balanced chemical equation
Determine the moles of the given substance using a mass-tomole conversion
Determine the moles of the unknown substance from the
moles of the given substance
From the moles of the unknown substance, determine the
mass of the unknown substance using a mole-to-mass
conversion
Limiting Reactants
 Why Reactions Stop
 One or more reactants are in excess and the reaction proceeds
until all of one reactant is used up
 Amount of product depends upon the reactant that is limited
 Limiting reactant – limits the extent of the reaction and
determines the amount of product
 Excess reactants – the other reactants that remain after the
reaction stops
 Calculating the Product When a Reactant is Limited
Determine which is the limiting reactant
1.

Find number of moles of each reactant
Determine if the reactants are in the correct mole ratio as
given in the balanced equation
3. After the limiting reactant has been determined, the amount
of product in moles can be calculated by multiplying the given
number of moles of the limiting reactant by the mole ratio
that relates the product and limiting reactant
4. Calculate the mass of the product by multiplying by the molar
mass of the product
2.
 Calculating the Excess Reactant
Determine amount that actually reacted by calculating the
mass of the non-limiting reactant needed to react completely
with the number of moles of the limiting reactant using a
mole-to-mass calculation
2. Multiply the moles of the limiting reactant by the mole ratio
of the reactants to obtain the number of moles of the nonlimiting reactant.
3. Obtain the mass of the non-limiting reactant
1.

Some reactions stop while portions of the reactants are still
present in the reaction mixture. B/c this is inefficient, excess
of one reactant is used (usually the least-expensive).
Percent Yield
 Many reactions stop before all of the reactants are used up, so
the actual amount of product is less than expected
 Theoretical yield – maximum amount of product that can be
produced form a given amount of reactant
 A reaction rarely produces this
 Actual yield – amount of product actually produced when
the chemical reaction is carried out in an experiment
 Percent yield – ratio of the actual yield to the theoretical
yield (expressed as %)
 Catalyst – substance that speeds a reaction but does not
appear in the chemical equation