Consider the reaction N₂ + 3H₂ → 2NH₃. A chemist mixes 2.0 mol N₂ with 3.0 mol H₂. Which is the limiting reagent?
AN₂, because it is present in fewer moles
BH₂, because it is present in more moles and will be consumed faster
CH₂, because 2.0 mol N₂ requires 6.0 mol H₂ but only 3.0 mol H₂ is available
DNeither — the reactants are in the correct stoichiometric ratio
The stoichiometric ratio requires 3 mol H₂ per mol N₂. For 2.0 mol N₂, you need 6.0 mol H₂, but only 3.0 mol is present — H₂ runs out first. Options A and B both make the common error of comparing raw amounts rather than checking against the required ratio. Option D is wrong: the ratio 2:3 ≠ 1:3 required by the equation.
Question 2 Multiple Choice
In an experiment producing aspirin, a student obtains 4.2 g of product. The theoretical yield calculated from the limiting reagent is 3.8 g. What is the most likely explanation?
AThe student was exceptionally skilled and exceeded the maximum possible yield
BThe product contains impurities or retained solvent, making its measured mass artificially high
CThe limiting reagent calculation was done correctly but percent yield can legitimately exceed 100%
DThe excess reagent contributed additional mass to the product
Percent yield cannot exceed 100% in a correctly conducted experiment — the theoretical yield is the absolute ceiling set by conservation of mass and stoichiometry. A yield above 100% invariably means the product is not pure: residual solvent, unreacted reagents, or other impurities add mass. Options A and C both treat >100% yield as achievable, which is physically impossible. Excess reagent does not incorporate into the product.
Question 3 True / False
The limiting reagent in a reaction is typically the reactant present in the smallest number of moles.
TTrue
FFalse
Answer: False
This is one of the most common misconceptions in stoichiometry. The limiting reagent is determined by comparing mole amounts to the stoichiometric coefficients — not by comparing mole amounts to each other. A reaction requiring 3 mol of A for every 1 mol of B could have A as the limiting reagent even if you have 10 mol of A and only 1 mol of B, if 10 mol of A is insufficient to react with 1 mol of B at the required ratio. Always divide available moles by the stoichiometric coefficient and compare.
Question 4 True / False
Once the limiting reagent is fully consumed, the reaction stops even if other reactants remain in the flask.
TTrue
FFalse
Answer: True
By definition, the limiting reagent is what limits the reaction. When it is gone, there are no more molecules of that species to react — even abundant excess reagent cannot drive the reaction further. This is exactly why identifying the limiting reagent first is essential: all yield calculations must be based on it, not on the excess reagent.
Question 5 Short Answer
Why is it insufficient to simply compare the number of moles of each reactant to determine the limiting reagent? What must you compare instead, and why?
Think about your answer, then reveal below.
Model answer: You must compare each reactant's available moles to the amount required by the stoichiometric ratio (i.e., available moles divided by its coefficient), or equivalently, calculate how much product each reactant could produce if fully consumed. Raw mole amounts are meaningless without knowing the ratio in which reactants combine. A reaction requiring 1:3 A:B means having twice as many moles of B as A still leaves B as the potentially limiting reagent — the balanced equation's coefficients define the 'recipe,' and the limiting reagent is whichever ingredient runs short relative to that recipe.
The sandwich analogy makes this concrete: 3 slices of cheese and 10 slices of bread for sandwiches requiring 2 bread + 1 cheese — cheese limits you to 3 sandwiches despite being present in fewer pieces, because the recipe calls for 2 bread per cheese and bread is abundant.