Limiting reactant and percent yield are frequently tested together as one combined problem, since a percent yield calculation always needs a theoretical yield first, and that theoretical yield depends on correctly identifying the limiting reactant.
Finding the limiting reactant
The reactant with the smaller starting mass or volume is not automatically the limiting one. The only reliable method is running the stoichiometry calculation from each reactant separately, converting each to how much product it could make on its own, and comparing. Whichever reactant produces less product is the limiting reactant.
Worked example
Problem
Convert each to moles
Find how much NH3 each reactant could make on its own
Identify the limiting reactant
Calculate theoretical yield
Percent yield
If an actual lab experiment produced 29.8 g of NH3, percent yield is actual divided by theoretical, times 100: (29.8 ÷ 34.0) × 100 = 87.6%. Percent yield accounts for real world losses, like incomplete reactions or product lost during purification, that a theoretical calculation can't predict.
Where students actually lose points here
Assuming the smaller given amount is automatically limiting
28.0 g of N2 is a bigger number than 12.0 g of H2, but N2 is still the limiting reactant here. Only an actual stoichiometry comparison, not the raw starting masses, can determine which reactant runs out first.
Calculating theoretical yield from the wrong reactant
Theoretical yield always comes from the limiting reactant specifically. Using the excess reactant's amount gives a theoretical yield that's too high.
Reporting percent yield over 100%
A percent yield over 100% signals an error somewhere, usually an impure product or a measurement mistake, since a reaction can't produce more product than the limiting reactant allows.
A frequent FRQ combination
Limiting reactant and percent yield show up together often enough that practicing them as one combined problem, not two separate skills, pays off directly on the exam.
Common questions
How do you know which reactant is limiting?
Convert each reactant's given amount to moles, then calculate how much product each one could make on its own using the mole ratio from the balanced equation. Whichever reactant produces less product is the limiting reactant.
Is the reactant with the smaller given mass always the limiting one?
Not necessarily. The only reliable way to determine the limiting reactant is running the stoichiometry calculation for each reactant separately and comparing the resulting product amounts.
What does it mean if percent yield comes out over 100%?
It signals an error somewhere in the experiment, commonly an impure product or a measurement mistake, since a reaction can never produce more product than the limiting reactant's theoretical yield allows.