So what the heck is Stoichiometry?

I have seen lots of definitions for this term– each more technical than the last. In essence, stoichiometry is the use of the ratio given in a balanced chemical equation. WHOA! He used the word “ratio.” That’s a scientific term, which breaks the rules of MPSN. This is supposed to be a site for people who just don’t get it. What the heck is this guy thinking?

Before you remove us from your bookmarks and badmouth us on forums, read on.

In an attempt to figure out how to boil this concept down to its most basic elements, I ran across Mr. Guch’s Great Big Vocabulary List. After laughing myself into convulsions, I realized that this is some pretty good stuff. You should check it out. But anyway, Mr. Guch defines stoichiometry as “The art of figuring how much stuff you’ll make in a chemical reaction from the amount of each reagent you start with.”

I couldn’t have said it better myself… so I didn’t. I instead quoted Mr. Guch. Is this becoming a little more clear now? Stoichiometry is figuring out how much A and how much B you need to get C… and then how much C you get. All this theory is starting to get a little cloudy, I’m sure. So let’s look at an example.

Take for example Aluminum Oxide. I’m sure that you can guess the two components of this compound: Aluminum and Oxygen.

Al + O₂ –> Al₂O₃

…but that’s not balanced…

4 Al + 3 O₂ –> 2 Al₂O₃

… now that’s balanced. But what does that really mean? If you’ve gotten this far in your study of chemistry, then you are familiar with the mole concept. That “4″ in front of the “Al” means that there are 4 moles of Al MOLECULES in this reaction. (It just kinda happens that a single aluminum atom can exist without being covalently bonded to any other atom.) The “3″ in front of the “O2″ means that there are 3 moles of O2 MOLECULES in this reaction. (Oxygen atoms, unlike aluminum atoms, cannot exist without being bonded to another atom. In this case, oxygen is bonded to another oxygen, thereby forming a gaseous oxygen molecule.) By now, you’ve already figured out that the “2″ in front of the “Al2O3″ means that the product is 2 moles of aluminum oxide.

BUT HOLD ON! Three moles plus four moles should give you SEVEN MOLES, not TWO!!! Sorry, but there’s no way to argue that. It’s just wrong. If you look more closely, you’ll see that the number of moles represents the number of moles of THAT MOLECULE, not the atoms that make it up. So, in this case, 4 moles of aluminum and 3 moles of oxygen only give us 2 moles of aluminum oxide.

Ok, so if you don’t learn ANYTHING ELSE FROM THIS ARTICLE, learn this concept. For every 4 moles of Al you put into the reaction, the most Al2O3 you can get out is 2 moles. For every 3 moles of O2 that you put into this reaction, the most Al2O3 you can get out is STILL 2 moles. THAT DOES NOT MEAN THAT YOU WILL ALWAYS GET 2 MOLES, it means that for every 4mol Al or 3mol O2, 2mol Al2O3 is the MAX that you can get in the end.

Stoichiometry Example Problem:

I’m going to give you 8 moles of Al and 3 moles of O2. What are your products going to be, and how much of each will you get?

Step 1: Limiting Reactant

Look at the balanced equation to see how many moles of product you could possibly get MAX from each of the reactants I gave you.

(8mol Al) x (2mol Al2O3 / 4mol Al) = 4 mol Al2O3

(3mol O2) x (2mol Al2O3 / 3mol O2) = 2 mol Al2O3

As you can see, although we have enough Aluminum to make 4 moles of product, we only have enough Oxygen to make 2 moles of product. So oxygen is our limiting reactant.

Step 2: Leftovers

That being said, all of the oxygen is going to be used up, and 4 moles of aluminum are going to be used up. (we know that just by looking at the balanced equation) This is going to leave you with 4 moles of Aluminum. So the final verdict is 4 moles of unreacted Al and 2 moles of Al2O3.

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Best of Luck,

Grey