Stoichiometry Calculator: Limiting Reagent, Theoretical & % Yield

If you're trying to figure out how many grams of product you can make from a reaction, stoichiometry is your answer. The most common mistake students make? Starting calculations without a balanced equation. Coefficients in that balanced equation become your conversion factors—they tell you the mole ratio between any two species. Without balanced coefficients, every number you calculate afterward is wrong.

Think of the mole map as a GPS for chemistry problems. You start with what you have (usually grams), convert to moles using molar mass, cross over to another species using the coefficient ratio, then convert back to grams. For 2 H₂ + O₂ → 2 H₂O, the coefficients say 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water. That's your roadmap.

Here's where units save you: grams → moles → moles → grams. Each arrow is a conversion factor. If your units don't cancel correctly, you've made a setup error. Write out every unit and cross them off as they cancel. This takes an extra 30 seconds but catches mistakes that cost you entire problems on exams.

The limiting reagent determines how much product you can actually make. It's the reactant that runs out first, stopping the reaction. Here's the mistake that costs points: assuming the reactant you have less of is limiting. That's not how it works. A tiny amount of one reagent might still be in excess if the stoichiometry requires even less.

To find the limiting reagent: convert each reactant to moles, divide by its coefficient, compare. The smallest result identifies your limiting reagent. For 2 Al + 3 Cl₂ → 2 AlCl₃ with 5 mol Al and 6 mol Cl₂: Al gives 5/2 = 2.5, Cl₂ gives 6/3 = 2.0. Chlorine is limiting because 2.0 < 2.5.

Everything else—theoretical yield, excess remaining, percent yield—depends on correctly identifying the limiting reagent. Get this step wrong and the entire problem cascades into nonsense.

Theoretical yield is the maximum product you could make if everything went perfectly—no side reactions, no losses, 100% efficiency. In reality, you never get theoretical yield. But you need to calculate it first to evaluate how well your reaction actually performed.

Start from the limiting reagent. Convert its moles to product moles using the coefficient ratio, then to grams using product molar mass. For N₂ + 3 H₂ → 2 NH₃ with H₂ limiting at 4 mol: product moles = 4 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 2.67 mol NH₃. Product mass = 2.67 mol × 17.03 g/mol = 45.4 g NH₃.

Notice the setup: you always multiply by a ratio where the thing you want is on top. If you're going from H₂ to NH₃, the NH₃ coefficient goes in the numerator. Mess this up and you get inverse answers.

After running a reaction in lab, you weigh your actual product. Percent yield compares this to theoretical yield: (actual / theoretical) × 100%. Typical organic synthesis yields run 60-85%. Getting 95%+ suggests either excellent technique or measurement errors.

Yields over 100% are red flags. Either your product is impure (contains solvent, byproducts, or starting material), your theoretical yield calculation is wrong, or you weighed something incorrectly. Before assuming you're a chemistry genius, recheck your numbers.

Low yields have causes: incomplete reaction (not enough time or heat), side reactions consuming reactant, product lost during transfer or purification, equilibrium not favoring products. For reversible reactions, Le Châtelier's principle explains why you can't reach 100%—some reactant always remains at equilibrium.

Problem: You have 25.0 g of Fe and 30.0 g of O₂. How many grams of Fe₂O₃ can you make? Equation: 4 Fe + 3 O₂ → 2 Fe₂O₃

Final answer: 35.8 g Fe₂O₃ theoretical yield. Notice how we used iron's moles throughout because it was limiting. The oxygen moles only mattered for identifying which reagent runs out first.

All stoichiometry rests on one principle: coefficients give mole ratios, and moles are the bridge between mass and particles. Grams mean nothing until you convert to moles because atoms combine by count, not by weight. A 1:1 mole ratio doesn't mean equal masses—it means equal numbers of formula units.

For solutions, use n = M × V where M is molarity (mol/L) and V is volume in liters. For gases at standard conditions, use n = PV/RT where R = 0.08206 L·atm/(mol·K). These convert your measurements into moles, which then plug into the standard stoichiometry workflow.

The units guide you. Set up each step so units cancel, leaving only what you want. If you end up with mol²/g or some other weird unit, your setup is wrong. Dimensional analysis isn't just a technique—it's error detection built into the method.