How to Use This Percent Yield Calculator
- Limiting reactant and yield: enter the coefficients from your balanced equation, each reactant’s formula and amount, and the product’s formula. Add an actual yield to get the percent yield too.
- Percent yield only: enter any two of actual yield, theoretical yield, and percent yield, and the calculator finds the third.
Formulas can include parentheses and hydrates, such as Ca(NO3)2 or CuSO4·5H2O. If you already know a molar mass, type the number instead of a formula. The equation has to be balanced first; if it is not, start with balancing the equation.
The Percent Yield Formula
Percent yield = (actual yield ÷ theoretical yield) × 100
- Theoretical yield is the most product the limiting reactant can make, from the balanced equation.
- Actual yield is the amount you collect and weigh in the lab.
How the Calculator Finds the Limiting Reactant
- Convert each reactant’s mass to moles using its molar mass.
- Use the coefficients to find how many moles of product each reactant could make on its own.
- The reactant that makes less product is limiting, and that amount is the theoretical yield.
- The other reactant is in excess; the leftover amount is what remains after the limiting reactant is used up.
Comparing grams directly gives the wrong answer, because reactants combine by moles. The full method, including the leftover excess and how it is scored, is worked through in AP Chemistry limiting reactant and percent yield.
Worked Example: Aluminum and Chlorine
2Al + 3Cl2 → 2AlCl3, starting with 10.0 g of Al and 35.0 g of Cl2. The lab collects 38.5 g of AlCl3.
| Step | Aluminum | Chlorine |
|---|---|---|
| Moles | 10.0 ÷ 26.98 = 0.3706 mol | 35.0 ÷ 70.90 = 0.4937 mol |
| AlCl3 it could make | 0.3706 × 2/2 = 0.3706 mol | 0.4937 × 2/3 = 0.3291 mol |
| Result | In excess | Limiting |
Theoretical yield: 0.3291 mol × 133.33 g/mol = 43.9 g AlCl3.
Aluminum left over: 0.3291 mol of Al is used (8.88 g), so 10.0 − 8.88 = 1.1 g remains.
Percent yield: 38.5 ÷ 43.9 × 100 = 87.7%.
Aluminum had the smaller mass but was not limiting, which is exactly why the comparison has to be made in moles.
Why Percent Yield Is Usually Below 100%
- Some product is lost when it is filtered, transferred, or rinsed.
- The reaction may not go to completion, or side reactions make a different product.
- A yield above 100% usually means the product was still wet or impure when it was weighed.
Comparing a measured value with an expected one is the same idea behind AP Chemistry percent error, which measures accuracy rather than how much product you recovered.
How the Calculator Handles Molar Masses and Sig Figs
Molar masses come from standard atomic masses, rounded to two decimal places for most elements as on the AP exam periodic table, so H2O is 18.02 g/mol. The theoretical yield is reported with the significant figures of the limiting reactant’s amount; coefficients are exact and never limit the answer. Leftover excess follows the subtraction rule, and percent yield uses the fewer significant figures of the two yields. To check any of these by hand, use the sig fig calculator, and for the mole conversions themselves see AP Chemistry molar mass. To see how stronger free-response answers could move your overall estimate, try the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- AP Chemistry Limiting Reactant & Percent Yield
- AP Chemistry Stoichiometry
- AP Chemistry Molar Mass
- AP Chemistry Balancing Equations
- Sig Fig Calculator
- Unit 4 Review: Chemical Reactions
Frequently Asked Questions
What is the formula for percent yield?
Percent yield = (actual yield ÷ theoretical yield) × 100. Both yields must be in the same unit, usually grams. If 38.5 g of product forms when 43.9 g was possible, the percent yield is 87.7%.
How do you find the limiting reactant?
Convert each reactant to moles, then use the mole ratio from the balanced equation to find how much product each one could make. The reactant that makes less product is the limiting reactant, and that smaller amount is the theoretical yield.
Can percent yield be over 100%?
Not for a pure, dry product. A percent yield above 100% usually means the product still contained water or another impurity when it was weighed, or that a measurement was wrong.
What is the difference between theoretical yield and actual yield?
The theoretical yield is the maximum amount of product the limiting reactant can make, calculated from the balanced equation. The actual yield is the amount you really collect and measure in the lab, which is usually less.
Is the limiting reactant the one with the smallest mass?
Not necessarily. What matters is moles compared through the coefficients of the balanced equation, not grams. A reactant with a larger mass can still run out first if its molar mass is large or the equation needs more of it.
This page is not affiliated with or endorsed by College Board. AP® is a trademark registered by the College Board.