Stoichiometry has a reputation for being the hardest math in introductory chemistry, but it's really the same four steps every single time, just with different starting and ending units. Once the pattern is automatic, the "hard" part disappears.
The 4 step method
Balance the equation
Mole ratios only mean something once the equation is balanced. Skipping this step, or balancing it wrong, throws off everything that follows.
Convert what you're given into moles
If you're given grams, divide by molar mass. If you're given liters of a solution, multiply by molarity. Moles are the currency stoichiometry actually runs on.
Use the mole ratio from the balanced equation
Multiply by the ratio of coefficients between what you have and what you want, target substance over given substance.
Convert moles of the target into whatever unit was asked for
Grams, liters, particles, whatever the question wants. This is just the reverse of step 2, applied to the new substance.
Worked example
Problem
Step 2: grams to moles
Step 3: mole ratio (2 mol H2O per 2 mol H2, a 1:1 ratio)
Step 4: moles to grams
When two reactant amounts are given: limiting reactant
If a problem gives you the starting amount of more than one reactant, it's testing limiting reactant. Run the 4 step method separately using each reactant to see how much product each one could make on its own. Whichever reactant produces less product is the limiting reactant, and that smaller amount is the actual answer.
Where students actually lose points here
Skipping the balancing step
An unbalanced equation gives a mole ratio that looks reasonable but is simply wrong, and every step built on top of it inherits that error.
Using the mole ratio upside down
The ratio has to be set up as target substance over given substance so the given substance's units cancel out. Flipping it gives an answer that's off by a predictable, easy to catch factor if you check units.
Forgetting to check for a limiting reactant
If a problem gives two starting amounts and only one is used in the calculation, the limiting reactant check got skipped, and the answer is very likely wrong.
The most repeated skill in the course
Stoichiometry isn't its own isolated topic, it's the calculation underneath titrations, gas law problems, and thermochemistry for the rest of the year.
Common questions
How do you know which reactant is limiting?
Calculate how much product each reactant could make on its own using the 4 step stoichiometry method. Whichever reactant produces less product is the limiting reactant.
Do you need grams or moles to start a stoichiometry problem?
Either works. If you're given grams, volume, or particles, the first step is always converting that into moles before using the mole ratio from the balanced equation.
What is percent yield?
Percent yield compares the actual amount of product made in a real reaction to the theoretical amount calculated through stoichiometry, expressed as a percentage: actual yield divided by theoretical yield, times 100.