Stoichiometry is the area of chemistry which deals with the
quantitative relationship among reactant and products. We will
use a unit analysis of moles and molar mass to solve problems in
stoichiometry. A mol-ratio is a ratio between any two species in
a chemical equation.
For example in the equation
4KO2(s) + 2CO2(g)
---> 2K2CO3(s) + 3O2(g)
there are a number of relationship implicit in the balanced
equation. For example, the equations says that 4 moles of KO2
reacts with 2 moles of carbon dioxide to produce 2 mol of
potassium carbonate and 3 moles of oxygen. The ratio of these
amount is always maintained in a chemical reaction. Beginning
with 2 moles of potassium peroxide require 1 mol of carbon
dioxide for complete reaction. 12 moles of KO2
requires 6 mol of carbon dioxide. The ratio is always maintained.
This ratio can be expressed from the balanced
chemical equation as the following unit conversion,
The unit conversion comes from the balanced chemical equation
and can be used to determine amounts required for reaction. For
example to determine how many moles of carbon dioxide are
required to react with 2 moles of potassium superoxide we could
perform the following conversion,
There are several additional mol ratios which derive from the
equation, a few include,
These ratios are used as conversion factors to convert from
moles of one substance to the corresponding moles of another
substance in a chemical rxn. Learning to do stoichiometric
calculations will require practice. One nice thing about these
calculations is the step-by-step procedure that can be followed
to arrive at an answer. The step-by-step procedure is as follows;
Balance the chemical equation which describes the
chemical reaction.
Use the molar mass of the reactant to convert the
grams of reactant to moles.
Use the unit conversion (mole ratio) from the
balanced chemical equation to convert from moles reactant
to moles product.
Use the molar mass of the product to convert from
moles to grams.
Lets demonstrate a variety of stoichiometric calculations
that you will be responsible for by beginning with several
straight forward uses of mol-ratios.
Sample Problem #1:
In the laboratory one might weigh amounts using the balance.
Let's consider another example,
Sample Problem #2:
Sample Problem #3:
Sample Problem #4: