What is "stoichiometry" (besides just a cool word to say), and why should I care about it?
The word "stoichiometry" comes from two Greek words: "stoicheion" (element) and "metron" (measure). It is the scientific method of determining the relative measures of the various elements that take part in a chemical reaction.
If all of the compounds that go into a reaction are in a "stoichiometric ratio", then there is no "extra" of any of the reactants: if they react as much as possible, then there will be none of the original chemicals left over.
This is important for two reasons: First, because we want to avoid wasting chemicals, and second, because the reaction will proceed as quickly and predictably as possible when the reactants are thoroughly pre-mixed in a stoichiometric ratio.
To find the stoichiometric ratio, we first work out a balanced chemical equation, and then work out the mass ratios using the atomic weights of the elements involved.
For instance, looking at the reaction of Hydrogen and Oxygen to form water:
H + O --> H2O
First problem: Hydrogen gas is always found as H2, not as monatomic hydrogen, and the same holds true for oxygen, as well (O2 instead of just O).
We need to fix the reaction formula to reflect that:
H2 + O2 --> H2O
The next problem: Too many Oxygens on the left side. (O2 has 2, and the O in H2O is just 1). So, we double the entire water molecule:
H2 + O2 --> 2H2O
New problem: too much Hydrogen on the right side. 2H2O has 4 hydrogen, so we need to double the hydrogen molecule on the left side:
2H2 + O2 --> 2H2O
This balances: 4 hydrogen atoms + 2 oxygen atoms yields 4 hydrogen atoms + 2 oxygen atoms.
So, we next look up the atomic weights (aka atomic masses) of the atoms involved. Use your periodic table (WebElements.com has a nice .pdf one you can print out). Hydrogen is about 1.0, and Oxygen is about 16.0.
So, for every 2 x (1.0 x 2) = 4.0 parts of Hydrogen, if you add (16.0 x 2) = 32.0 parts of oxygen, you'll end up with 2 x ( ( 1.0 x 2 ) + ( 16.0 ) ) = 36.0 parts of water.
The stoichiometric ratio of hydrogen to oxygen (H:O) by mass is 4:32 (or 1:8, if you simplify by dividing both numbers by 4). It doesn't matter whether you're talking grams, kilograms, pounds, or tons -- any 1:8 mass ratio of hydrogen:oxygen will react completely to form water.
Similar techniques apply to every chemical reaction, including thermite reactions:
3CuO + 2Al --> 3Cu + Al2O3
CuO + Mg --> Cu + MgO