"Stoichy-What?" (It just means "measuring elements")
Everybody calm down. Stoichiometry has a bad rap as being difficult even impossible, but really it's just a ONE step operation, and all you have to do is look at the coefficients in the balanced equation. The name stoichiometry comes from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). See? You're just measuring moles of elements!
So why all the fuss? Because to do stoichiometry you still need to use molar ratios, find molar masses, identify reactions, predict products, and balance equations, and convert between grams and moles. If you struggled with any of those in class, you're not alone, so just go back and review them if you need to. Bottom line, stoich seems scary because it ties EVERYTHING we've learned so far together. If you can't do the background stuff, you can't do stoichiometry.
It's All About Mole Ratios!
Stoichiometry is no harder than adjusting a recipe for making cookies (mmmm, cookies!). Doing the "stoichiometry" is just like scaling a recipe up or down depending on either how many ingredients (reactants) you have or how much food (product) you want to make. Let's say our recipe calls for one 12-ounce bag of chocolate chips for every 5 dozen cookies you want to make. Okay, how many bags of chips do you need to make 10 dozen cookies? Two, right? You double the recipe. That's stoichiometry!
Let's dip a toe in the stoichiometry pool and start with an easy and familiar example, making water. We know the balanced equation is 2H2 + O2 -> 2H2O.
What this means is "2 mol of hydrogen + 1 mol of oxygen makes 2 mol of water." The coefficients tell you how many mols of each reactant and product you have in a balanced chemical equation. If you want to make 4 moles of water, you need 4 moles of hydrogen and 2 moles of oxygen. If you want to make 1 mole of water you only need one mole of hydrogen and half a mole of oxygen. See? Easy!
Solving Basic Mol-Mol Stoich Problems
Stoichiometry Problems with Limiting and Excess Reactants
So far when we've been solving for how much product a reaction makes we've been solving for the "theoretical yield." Theoretical yield is the maximum product we could get in a perfect world if our reactants are available in perfect ratios. But - you guessed it - we don't live in a perfect world. Sometimes one reactant will run out and stop the reaction before all the reactants are used up.
We call the reactant that runs out first and stops the reaction the "LIMITING REACTANT." The reactant that's left over (wasted, really) unable to react is called the "EXCESS REACTANT."