Enter a balanced equation and how much of each reactant you have. This finds the limiting reactant, the theoretical yield, and the percent yield, with every mole conversion shown.
In N₂ + 3H₂ → 2NH₃, the numbers in front are a recipe in moles, not in grams. One mole of nitrogen reacts with three moles of hydrogen to give two moles of ammonia. That ratio is the whole basis of stoichiometry.
This is why you cannot work in grams directly. Grams do not follow the ratio, because different substances have different molar masses. You convert grams to moles, use the ratio, then convert back to grams at the end. Every stoichiometry question follows that same shape.
Reactions rarely start with exactly the right proportions. One reactant runs out first, and once it does the reaction stops regardless of how much of the others is left. That one is the limiting reactant, and it alone sets how much product you can make.
The way to find it is not to look for whichever you have least of by mass. Instead, divide each reactant's moles by its coefficient in the equation. The smallest result is the limiting one, because that comparison accounts for the recipe rather than just the raw amount.
Everything else is in excess. Some of it will be left over unreacted when the reaction finishes.