AP Chemistry · Exam guide
AP Chemistry Stoichiometry: Limiting Reactant and Percent Yield, Step by Step
Stoichiometry questions are a chain: grams to moles, moles to moles with the coefficients, and moles to grams. The questions get harder only when two reactants are given, so you must find which one limits. This guide shows the chain on one example.
The mole map
Use the balanced equation. Convert each quantity to moles, compare using the coefficients, and convert the answer to the units asked for.
A worked example
Nitrogen and hydrogen react to make ammonia: \(\mathrm{N_2} + 3\,\mathrm{H_2} \rightarrow 2\,\mathrm{NH_3}\). A mixture contains 28.0 g of \(\mathrm{N_2}\) and 5.00 g of \(\mathrm{H_2}\). (a) Which reactant is limiting? (b) How many grams of \(\mathrm{NH_3}\) can form? (c) If 22.5 g of \(\mathrm{NH_3}\) is actually collected, what is the percent yield?
Molar masses: \(\mathrm{N_2}\) is 28.02 g/mol, \(\mathrm{H_2}\) is 2.016 g/mol, and \(\mathrm{NH_3}\) is 17.03 g/mol.
(a) Moles of \(\mathrm{N_2}\) = 28.0 / 28.02 = 0.999 mol. Moles of \(\mathrm{H_2}\) = 5.00 / 2.016 = 2.48 mol. The reaction needs 3 mol of \(\mathrm{H_2}\) for each mole of \(\mathrm{N_2}\), so 0.999 mol of \(\mathrm{N_2}\) would need 3.00 mol of \(\mathrm{H_2}\), and only 2.48 mol is present. Hydrogen is the limiting reactant.
(b) \(2.48\ \mathrm{mol\ H_2} \times \dfrac{2\ \mathrm{mol\ NH_3}}{3\ \mathrm{mol\ H_2}} = 1.65\ \mathrm{mol\ NH_3}\), and \(1.65 \times 17.03 = 28.2\) g.
(c) Percent yield \(= \dfrac{22.5}{28.2} \times 100 = 79.9\%\).
Mass is not the comparison. The reactants must be compared as moles, using the coefficients.
It compares moles using the mole ratio.
What graders look for
- A balanced equation that you use.
- Units on every number so that the cancellation is visible.
- The limiting reactant found by comparing moles with the coefficients.
- The yield based on the limiting reactant, never on the one in excess.
- Percent yield as actual divided by theoretical.
Common mistakes
- Using an unbalanced equation.
- Comparing grams of reactants instead of moles.
- Basing the theoretical yield on the reactant in excess.
- Dividing theoretical by actual yield, which gives a value above 100 percent.
Frequently asked questions
How many significant figures should I give?
Match the least precise data in the problem. Here, three.
Is the limiting reactant always the one with fewer moles?
No. It is the one that runs out first after accounting for the coefficients.
Can percent yield be above 100 percent?
In a real experiment, only if the product is impure or wet. It signals an error in a problem like this one.
Do I need to show units?
Yes. Unit cancellation is part of the setup that earns credit.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every lesson in the AP Chemistry course ends with practice problems and an Exam focus box that shows how the topic is tested, and every unit has a review with worked answers. These free lessons also build the skills the free-response section tests:
See the AP Chemistry course Free lessons and lesson list
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Multiple choice
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