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6.S: Quantitative Relationships in Chemistry (Summary)
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Stoichiometry
is the relationship between the masses of chemical reactants and products in a given chemical reaction. The coefficients placed in a chemical equation in order to balance it are called the
stoichiometric coefficients
.
Molar stoichiometry
is simply the expression of the coefficients of a reaction in terms of moles of reactants and products.
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In order to find the number of
moles of a product
that is produced in a chemical reaction when you are
given
moles of reactant
, simply multiply the moles of reactant by the
stoichiometric ratio
relating that reactant and the desired product; i.e., \[(molreactant)\times \left ( \frac{molproduct}{molreactant} \right ) \nonumber \]
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In order to find the number of
moles of a product
that are produced in a chemical reaction when you are
given
mass of reactant
, simply
divide
the mass of reactant by the
molar mass
(to get
moles reactant
) and then
multiply
by the
stoichiometric ratio
relating that reactant and the desired product; i.e., \[\left ( \frac{grams}{grams/mol} \right )\times \left ( \frac{molproduct}{molreactant} \right ) \nonumber \]
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Always remember,
mass divided by molar mass equals moles
; \[\left ( \frac{grams}{grams/mol} \right )=mol \nonumber \]
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The
mass
or the
number of moles
that you calculate for a product based on reaction stoichiometry is called the
theoretical yield
for the reaction. The amount of material that you actually isolate from a given reaction is called the
actual yield
and it is always less than the theoretical yield. The ratio of the actual and theoretical yields, expressed as a
percentage
is called the
percentage yield
.
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If
a reaction requires more than one reactant and
if
you are given the mass, or the number of moles of
each
reactant, you must approach the calculation as a
limiting reactant problem
. To solve a limiting reactant problem, simply perform the standard mass calculation for
each
reactant, noting the mass (or number of moles) of product formed in each calculation. The reactant that yields the
smallest
amount or product from these calculations is called the
limiting reactant
. Reactants that yield
larger
amounts of products in these calculations are called
excess reactants
. The theoretical yield in the reaction will be based
solely
on the calculated amount for the limiting reactant.
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If a reactant in a chemical reaction is said to be “in excess”, you assume that you have
unlimited
amount of the reactant, and that it will never be the limiting reactant.