limiting reagent
/ LIM-it-ing ree-AY-jent /
If you have ten slices of bread but only three slices of cheese, you can make at most three sandwiches — the cheese runs out first and caps the whole batch. The limiting reagent is the cheese of a chemical reaction.
The limiting reagent is the reactant that is completely consumed first, thereby setting the maximum amount of product the reaction can form. You identify it by comparing how many moles of each reactant you have against the mole ratios in the balanced equation, not by which one weighs less.
Spotting it is essential for predicting yield: every theoretical-yield calculation must be based on the limiting reagent, never the one left over in excess. The leftover reactant, in excess, simply remains unreacted and may complicate purification afterwards.
Burning 2 moles of hydrogen with only 0.5 mole of oxygen (which needs a 2:1 ratio) means oxygen runs out first; oxygen is the limiting reagent and only 1 mole of water can form.
The reactant that runs out first caps the yield.
The limiting reagent is decided by mole ratios, not by raw mass or volume. A reactant you have a lot of by weight can still be limiting if its molar mass is large or the equation demands many of its moles.