Percentage, and Equation Stoichiometry
In many equation stoichiometry problems, we can assume that the liquids and solids in our
reaction mixtures were pure. Unfortunately (for chemistry students), the
real world is physically quite complex, and chemists must often work
with substances that are part of a mixture. For example, apatite, Ca5(PO4)3F,
which is a common reactant used to make a number of important compounds,
is often added to the reaction vessel in the form of apatite ore, which
is a mixture of Ca5(PO4)3F with other
substances. The amount of Ca5(PO4)3F in
the ore is usually described in terms of percentage. A typical apatite
ore has over 75% Ca5(PO4)3F.
When one of the substances needed for a reaction
is part of a mixture, our equation stoichiometry problems require a new step to
convert from the mass of the mixture to the mass of the component in it that is
needed for the reaction. The conversion factor needed for this step comes from
the substances mass percentage.
The conversion factor that comes from the
percentage can also be used at the end of an equation stoichiometry problem to
convert from mass of a substance to mass of mixture that contains the substance.