Limiting Reagent Calculator
Find the limiting reagent, theoretical yield, excess left over and percent yield from the masses, molar masses and coefficients of a reaction.
What this calculator does
Enter the mass, molar mass and coefficient of two reactants and the product. The calculator finds which reactant runs out, the theoretical yield, how much of the other is left and the percent yield.
When you need it
In a real reaction one reactant is nearly always used up first. Finding it tells you the maximum yield, how much excess you wasted and how to read your percent yield.
Formulas
- n = m ÷ M for each reactant
- Equivalents = n ÷ coefficient; the smaller value is the limiting reagent
- Percent yield = (actual yield ÷ theoretical yield) × 100
How to use it
- Enter mass, molar mass and coefficient for reactants A and B.
- Enter the molar mass and coefficient of the product.
- Optionally enter the actual yield to get the percent yield.
- Read the limiting reagent, the theoretical yield and the excess.
Worked example
N2 + 3 H2 → 2 NH3. Which reagent is limiting for 28.0 g N2 and 8.00 g H2, and how much NH3 forms?
- n(N2) = 28.0 ÷ 28.02 = 0.999 mol; n(H2) = 8.00 ÷ 2.016 = 3.97 mol
- Equivalents: N2 0.999 ÷ 1 = 0.999; H2 3.97 ÷ 3 = 1.32
- N2 has the smaller value, so N2 is limiting
- n(NH3) = 0.999 × 2 = 2.00 mol; mass = 2.00 × 17.03 = 34.0 g
- H2 used = 3 × 0.999 = 3.00 mol, so 0.97 mol (1.96 g) of H2 is left over
Answer
N2 is the limiting reagent. The theoretical yield is 34.0 g of NH3, with 1.96 g of H2 left over.
Common mistakes
- Comparing moles directly without dividing by the coefficients.
- Calculating the yield from the reagent in excess.
- Reporting percent yield above 100% without checking for impure product or a mistake.
Related tools
Frequently asked questions
What is a limiting reagent?
The reactant that runs out first, so it limits how much product can form.
How do you find the limiting reagent?
Convert both masses to moles, divide each by its coefficient, and pick the smaller result.
What is percent yield?
The actual amount of product obtained divided by the theoretical yield, times 100.