Divide what you have by what the recipe needs. The smallest answer wins.
A balanced equation is a recipe. To find the limiting reactant, convert each reactant to moles and divide by its coefficient. That gives you how many "batches" of the recipe each reactant could support. Whichever supports the fewest batches is limiting, and it sets the theoretical yield.
Run a batch
Limiting reactant problems
Percent yield is yield reconciliation.
Theoretical yield is what the chemistry allows. Actual yield is what makes it to the scale. The gap is never zero: side reactions, product left on equipment, line flushes, sampling, and incomplete reactions all take a cut. A process engineer's job is mostly finding where that gap goes.
Industry also uses excess reactant on purpose. In the Haber process (N₂ + 3H₂ → 2NH₃), and in most combustion, the cheaper reactant is fed in excess to push the expensive or hazardous one to react completely. Burners run with excess air so no unburned fuel or carbon monoxide leaves the stack.
Common mistakes
- Picking the reactant with the smaller mass, or fewer moles, as limiting. Divide by the coefficient first.
- Calculating the theoretical yield from the excess reactant. Always use the limiting one.
- Getting a percent yield over 100% and accepting it. That means the product is wet or impure, or the math is off.
- Forgetting that the excess left over = what you started with − what reacted (found from the limiting reactant's moles and the mole ratio).