Stoichiometry
Start here if stoichiometry feels like too many steps at once. This unit shows you the one idea underneath all of it: use the balanced equation to move through mole ratios, then convert to the unit the question wants — preparing you for bonding.
What you'll learn
9.1 Start Here: What Stoichiometry Is Actually Doing
The word stoichiometry (stoy-key-AH-meh-tree) sounds intimidating, but the idea is simple. It is the math that tells you how much of each chemical is used or made in a reaction.
This is not a new idea disconnected from the last two units. It combines the mole work from Unit 07 with the balanced equations from Unit 08.
Think of a recipe. If a cookie recipe says "use 2 cups of flour for every 1 cup of sugar," you can scale it up or down.
A balanced chemical equation works the same way. The coefficients act like the recipe amounts for the reaction.
This means: 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to make 2 molecules of water. The ratio 2 : 1 : 2 never changes — no matter how big or small the batch.
The coefficient says each batch needs 2 moles of hydrogen.
The same batch needs 1 mole of oxygen.
That batch makes 2 moles of water.
Big idea
- The coefficients in a balanced equation tell you the mole ratio — the ratio of moles of every substance in the reaction.
- This ratio is the heart of all stoichiometry problems.
- If the equation is not balanced first, every later number is built on the wrong ratio.
9.2 Mole Ratios: The Conversion Tool That Drives the Whole Unit
A mole ratio (also called a stoichiometric factor) is a fraction you build from the coefficients in a balanced equation. You use it like a unit conversion — it lets you switch from moles of one substance to moles of another.
Notice what usually goes wrong here: grabbing numbers from subscripts or molar masses instead of the equation. Do not do that. The mole ratio comes only from the coefficients in the balanced equation.
From the equation N₂ + 3 H₂ → 2 NH₃, you can write these mole ratios:
Do not miss this
- You can flip any mole ratio upside down.
- Pick the version that cancels the unit you want to get rid of.
- The substance you start with goes in the denominator. The substance you want goes in the numerator.
- This is the same factor-label method you used in the Moles unit.
9.3 The Stoichiometry Roadmap: What to Do Step by Step
Many stoichiometry problems follow this path when you start with grams and need grams of a different substance. First identify what you are given and what you are asked to find. Then use only the steps you need.
Always check that the equation is balanced before using any mole ratio. If the coefficients are wrong, every later conversion will also be wrong.
If this feels shaky, slow down and label the path before you calculate. Most mistakes in stoichiometry happen before the arithmetic even starts.
Common grams-to-grams path:
- Step 1 — Convert to moles: Divide the grams given by the molar mass of that substance. Now you have moles.
- Step 2 — Use the mole ratio: Multiply by the stoichiometric factor from the balanced equation. This switches you from moles of the given substance to moles of the wanted substance.
- Step 3 — Convert to grams: Multiply by the molar mass of the wanted substance. Now you have grams of your answer.
Where MM = molar mass in g/mol, and the middle fraction is the mole ratio from the balanced equation.
If you start with moles, skip Step 1. If your answer should be in moles, stop after Step 2.
9.4 Limiting Reactant: Which Reactant Runs Out First?
What happens when you do not have exactly the right amounts of each reactant? One ingredient runs out first and stops the reaction. That ingredient is called the limiting reactant (or limiting reagent).
Start here if you keep mixing up limiting and excess reactants. The limiting reactant is not the one with the smaller starting mass. It is the one that can make less product once the balanced equation is taken seriously.
In chemistry, the limiting reactant controls how much product you can make. Once it is used up, the reaction stops — even if other reactants are left over.
Compare raw grams and pick the smaller number.
That fails because different substances have different molar masses and different coefficients.
Convert each reactant through the balanced equation and ask how much product each could make.
The reactant that makes less product is limiting.
Common mistake
- The instinct is to pick the reactant with fewer grams.
- That is wrong — you must compare moles relative to the equation, not raw grams.
- Always convert to moles first.
9.5 Theoretical Yield, Actual Yield, and Percent Yield
Once you know the limiting reactant, you know the maximum amount of product the reaction could make. That prediction is the theoretical yield. What you actually collect in the lab is usually smaller.
Here are the three terms and what each one refers to. They build on each other — you need the theoretical yield before you can calculate percent yield.
Actual yield goes on top because it is the amount you got. Theoretical yield goes on the bottom because it is the maximum predicted amount.
If your answer is above 100%
- True chemical yield should not exceed 100%, but a measured value above 100% can happen if the product is wet, impure, or measured incorrectly.
- If your calculation gives more than 100%, something went wrong — recheck your math or your measurement.
Next step after Unit 09
Stoichiometry is the main quantity tool for the rest of chemistry. Once these setups feel solid, move into chemical bonding for the next big content shift, and come back to stoichiometry whenever reaction quantities appear again.