Chemical reactions provide two important pieces of information.
  1. They tell us the type and number of atoms and molecules that interact and how they arrange.
  2. They tell us the relative number of moles of atoms and molecules that interact and form.
Let us examine a chemical reaction to show how this is true.
Reaction ----->
1 H2
+
1 Cl2
2 HCl
The reaction indicates that if we had 1 molecule of H2, then
1 molecule of H2
+
1 molecule of Cl2
2 molecules of HCl
These are relative ratios therefore, if we had 10 molecules
10 molecules
+
10 molecules
20 molecules
If we had 1,000 molecules,
1,000 molecules
+
1,000 molecules
2,000 molecules
If we had a mole of Hydrogen then
1 mole
+
1 mole
2 mole
As you learned in Lesson #6, moles can be converted to masses, volumes or particles, therefore we can use a chemical reaction to calculate how much of a substance will form or react. Stoichiometry is the name we give to the process of using a chemical reaction to predict the amount of products that might form or reactant that might be used up.