Applying Conversion Factors to Stoichiometry
Now you're ready to use what you know about conversion factors to solve some
stoichiometric problems in chemistry. Almost all stoichiometric problems can be
solved in just four simple steps:
- Balance the equation.
- Convert units of a given substance to moles.
- Using the mole ratio, calculate the moles of substance yielded by the
reaction.
- Convert moles of wanted substance to desired units.
These "simple" steps probably look complicated at first glance, but relax, they
will all become clear.
Let's begin our tour of stoichiometry by looking at the equation for how iron
rusts:
Step 1. Balancing the Equation
The constituent parts of a chemical equation are never destroyed or lost: the
yield of a reaction must exactly correspond to the original reagents. This fact
holds not just for the type of elements in the yield, but also the number.
Given our unbalanced equation:
This equation states that 1 iron (Fe) atom will react with two oxygen (O) atoms
to yield 2 iron atoms and 3 oxygen atoms. (The subscript number, such as the two
in
O2 describe how many atoms of an element are in a molecule.) This
unbalanced reaction can't possibly represent a real reaction because it
describes a reaction in which one Fe atom magically becomes two Fe atoms.
Therefore, we must balance the equation by placing coefficients before the
various molecules and atoms to ensure that the number of atoms on the left side
of the arrow corresponds exactly to the number of elements on the right.
Let's count up the atoms in this new, balanced version of the reaction. On the
left of the arrow we have 4 atoms of iron and 6 atoms of oxygen (since
3×2 = 6). On the right we also have 4 iron (since
2×2 = 4) and
6 oxygen (
2×3 = 6). The atoms on both sides of the equation match.
The process of balancing an equation is basically trial and error. It gets
easier and easier with practice. You will likely start to balance equations
almost automatically in your mind.
Step 2. Converting Given Units of a Substance to Moles
The process of converting given units into moles involves conversion factors.
Below we will provide the most common and important conversion factors to
convert between moles and grams, moles and volumes of gases, moles and
molecules, and moles and solutions. These conversion factors function in the
same way as those discussed in the previous
section
Note also
that though these conversion factors focus on converting from some other unit to
moles, they can also be turned around, allowing you to convert from moles to
some other unit.
