Stoichiometry is the use of a balanced chemical equation to predict exact amounts: how much product a reaction will make, or how much reactant it will take. Every stoichiometry problem follows the same four steps. Balance the equation, convert what you have into moles, apply the mole ratio from the coefficients, then convert into whatever unit the question asked for.
Stoichiometry is the math of chemistry. It is how we compare apples to oranges, or how we compare how much stuff we started with to how much stuff we produce. Here is everything you need to understand it from scratch.
What is the one idea stoichiometry is built on?
Strip away the vocabulary and stoichiometry is just this: a balanced chemical equation tells you the ratio in which substances react and form, and you can use that ratio to predict exact amounts. If you know how much of one substance you have, you can calculate how much of anything else in the reaction you will need or produce.
Everything else in this unit, mole ratios, limiting reactants, percent yield, is that one idea applied in different situations. That is genuinely all it is.
Step 1: why do you have to balance the equation first?
Every stoichiometry problem starts here, and skipping it is the single most common reason students get the wrong answer. An unbalanced equation gives you the wrong ratio, and the wrong ratio makes every calculation after it wrong too, even if your math is flawless.
If balancing is still slow for you, fix that before anything else. Our guide on how to balance chemical equations walks through three methods.
Step 2: how do you convert what you have into moles?
Every chemistry problem starts somewhere. Maybe you were told how many grams or milligrams of a substance were present, maybe how many particles were reacting. That is your starting point, your Whatcha Got.
Chemists work in moles because moles connect directly to the coefficients in a balanced equation. Grams do not. So the goal of this step is always the same, get to moles:
- Given a mass? Divide by the substance’s molar mass.
- Given volume and concentration of a solution? Multiply them. Molarity times liters gives moles.
- Given a gas at known conditions? Use the ideal gas law, PV = nRT, and solve for n.
- Given a number of particles? Divide by Avogadro’s number.
If moles themselves are still shaky, The Mole and the Mole City episodes that follow it are built for exactly this.
Step 3: how does the mole ratio work?
Once you have moles of your known substance, the Whatcha Got, use the coefficients from the balanced equation to find moles of whatever you are solving for, the Whatcha Need.
If your equation reads 2A + B gives 3C, and you have 4 moles of A, multiply by the ratio of 3 moles C over 2 moles A to find you can produce 6 moles of C.
This ratio step is the actual heart of stoichiometry. Everything before it is just getting you to moles, and everything after it is just converting back out. The only genuinely new step in any stoichiometry problem is the mole ratio, the moment you compare two different substances.
Step 4: how do you convert back into the units the question wants?
Now convert your answer in moles into whatever unit the question actually asked for: grams, liters, particles, or concentration. This is the same conversion you did in Step 2, just running in reverse.
Read the question one more time before you circle anything. Students lose real points every year by solving perfectly and then handing in moles when the question asked for grams.
A full worked stoichiometry example
How many grams of water form when 10 grams of hydrogen gas reacts completely with oxygen?
- Balance. 2H2 + O2 gives 2H2O.
- To moles. 10 g of H2 divided by 2.02 g/mol is about 4.95 moles of H2.
- Mole ratio. 2 moles of H2O for every 2 moles of H2, a 1 to 1 ratio, so 4.95 moles of H2O.
- Back out. 4.95 moles times 18.02 g/mol is about 89.2 grams of water.
Every stoichiometry problem, no matter how complicated it looks, follows this exact four step path. Solving Typical Stoichiometry works through more of these on video.
What is a limiting reactant, and how do you find it?
This is where most students get stuck. When you are given amounts of two reactants instead of one, you cannot assume both will be fully used up. One will run out first and stop the reaction. That is your limiting reactant, and it is the only one that determines how much product you actually get.
To find it, run the full stoichiometry calculation from each reactant separately and see which one predicts less product. Whichever gives the smaller amount is the limiting reactant, and that smaller amount is your real answer. The other reactant is in excess, meaning some of it is left over unreacted.
The signal to watch for is simple: any time a problem hands you starting amounts of two or more reactants, check for a limiting reactant. See Limiting Reactant Stoichiometry and Percent Yield.
Frequently asked questions
What is stoichiometry in simple terms?
Stoichiometry is using a balanced chemical equation as a recipe. The coefficients tell you the ratio in which substances react and form, and that ratio lets you calculate exactly how much product you will get from a given amount of reactant, or how much reactant you need for a target amount of product.
What are the four steps of a stoichiometry problem?
Balance the chemical equation. Convert the amount you were given into moles. Apply the mole ratio from the balanced coefficients to get moles of the substance you want. Convert that back into the unit the question asked for.
Why does stoichiometry use moles instead of grams?
Because the coefficients in a balanced equation are ratios of particles, not of mass. Two moles of hydrogen and one mole of oxygen react in a 2 to 1 particle ratio, but not a 2 to 1 mass ratio, since the atoms weigh different amounts. Moles are the unit that connects directly to the coefficients.
How do you know which reactant is limiting?
Calculate how much product each reactant would produce if it were fully consumed. The reactant that predicts the smaller amount of product is the limiting reactant, and that smaller amount is the actual yield the reaction can reach. The other reactant is in excess.
Is stoichiometry hard?
The math itself is straightforward, mostly multiplication and division. What makes it feel hard is that it stacks on top of two earlier skills, balancing equations and mole conversions, so a weakness in either one shows up as a stoichiometry problem you cannot solve. Fix the foundation and stoichiometry usually clicks quickly.
This four step process shows up in nearly every unit that follows, from thermochemistry to equilibrium, so it is worth being fully automatic with it. It is built step by step with guided practice problems in the full AP and Honors Chemistry course.