Untitled - Kelly Walsh High School

Mass, Moles, and Equations 35
Quick Tip
3-3 Moles and Stoichiometry
Careful!
Notice that if we had grams and wanted just particles, we still would need to
incorporate the mole relationship.
If you are starting with either grams or particles, you will need to calculate moles.
As we mentioned previously, the balanced chemical equation not only indicates
what chemical species are the reactants and what the products are, but it also
indicates the relative ratio of reactants and products. Consider the balanced
equation for the rusting of iron:
4 Fe(s) 3 O 2(g) l 2 Fe 2O 3(s)
This balanced equation can be read as 4 iron atoms react with 3 oxygen molecules
to produce 2 iron(III) oxide units. However, the coefficients can stand not
only for the number of atoms or molecules (microscopic level) but they can also
stand for the number of moles of reactants or products. So the equation can also
be read as: 4 mol of iron react with 3 mol of oxygen to produce 2 mol of iron(III)
oxide. In addition, if we know the number of moles, the number of grams or
molecules may be calculated. This is stoichiometry, the calculation of the
amount (mass, moles, particles) of one substance in the chemical equation from
another. The coefficients in the balanced chemical equation define the mathematical
relationship between the reactants and products and allow the conversion
from moles of one chemical species in the reaction to another.
Consider the rusting process above. How many grams of rust (Fe 2O 3) could
form in the reaction of 20.0 mol of iron with excess oxygen?
Before any calculations can be done, you must have a balanced chemical equation!!