The theoretical yield is the amount of product you would get if the reaction went to completion with no side reactions and no losses – the stoichiometric ceiling against which real (actual) yields are judged. It is set by the limiting reagent, the reactant that runs out first. Once it is used up the reaction stops, however much of the other reactants is left.
Finding the limiting reagent takes one step per reactant: convert its mass to moles, then divide by its coefficient in the balanced equation. The reactant with the smallest result is limiting. Multiplying that value by the product’s coefficient gives the moles of product, and multiplying by the product’s molar mass gives the theoretical yield in grams. Everything else is in excess, and the calculator tells you how much of each excess reactant remains.
Use the simple mode when you already know the limiting reagent and its coefficients, as in a textbook problem. Use the equation mode to type the reaction (balanced or not), list the amounts of all reactants in grams or moles, choose the product, and let the calculator balance the equation, find the limiting reagent and compute the yield. Add an expected percent yield to see the amount you are likely to isolate.
How to use the Theoretical Yield Calculator
- Method
- Mass of the limiting reagent
- Molar mass of the limiting reagent
- Coefficient of the limiting reagent
- Coefficient of the desired product
- Molar mass of the desired product
- Chemical equation — Balanced or not; use -> between the sides
- Amount of each reactant, in equation order — Reactants left off the end of the list are treated as being in excess
- Unit of these amounts
- Desired product
- Expected percent yield (optional)
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How it works
In the equation mode the equation is balanced, each reactant amount is converted to moles with IUPAC atomic weights, and moles ÷ coefficient is compared to find the limiting reagent. The product’s moles follow from the mole ratio and are converted to grams; excess reactant left over = moles supplied − moles consumed.
Worked example
Aspirin synthesis: C₇H₆O₃ (salicylic acid) + C₄H₆O₃ (acetic anhydride) → C₉H₈O₄ (aspirin) + C₂H₄O₂, already balanced 1 : 1 : 1 : 1. With 2.00 g of salicylic acid (2.00 ÷ 138.12 = 0.01448 mol) and 5.00 g of acetic anhydride (5.00 ÷ 102.09 = 0.04898 mol), salicylic acid has the smaller moles ÷ coefficient and is the limiting reagent.
Aspirin formed: 0.01448 mol × 180.16 g/mol = 2.61 g theoretical yield. Acetic anhydride left over: 0.04898 − 0.01448 = 0.0345 mol ≈ 3.52 g. At an 85% yield you would expect to isolate about 2.22 g.
Step-by-step method
- Write and balance the equation.
- Convert every reactant amount to moles (mass ÷ molar mass).
- Divide each by its coefficient – the smallest result is the limiting reagent.
- Multiply that number by the product coefficient to get moles of product.
- Multiply by the product molar mass to get the theoretical yield in grams.
Formula
n(limiting) = m ÷ M · n(product) = n(limiting) × ν(product) ÷ ν(limiting) · theoretical yield = n(product) × M(product) · limiting reagent = smallest n ÷ ν
Worked example
Inputs — Method: From a chemical equation – find the limiting reagent; Chemical equation: C7H6O3 + C4H6O3 -> C9H8O4 + C2H4O2; Amount of each reactant, in equation order: 2, 5; Unit of these amounts: g; Desired product: C9H8O4; Expected percent yield (optional): 85 %.
Results — Theoretical yield = 2.6087; Theoretical yield in moles = 0.01448; Limiting reagent = C₇H₆O₃ (runs out first; 0.01448 mol of reaction); Balanced equation = C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + C₂H₄O₂; Moles of the limiting reagent = 0.01448; Expected actual yield at your percent yield = 2.2174.
Key terms
- Theoretical yield
- The maximum product predicted by stoichiometry from the limiting reagent.
- Limiting reagent
- The reactant consumed first, which stops the reaction.
- Excess reagent
- A reactant left over when the limiting reagent is used up.
- Mole ratio
- The ratio of coefficients that converts moles of one species into moles of another.
Frequently asked questions
What is theoretical yield?
The maximum amount of product that can form from the given reactants, calculated from the limiting reagent and the balanced equation, assuming complete reaction and no losses.
How do I calculate theoretical yield?
Convert the limiting reagent to moles, multiply by (product coefficient ÷ reagent coefficient), then by the product's molar mass. 2.00 g of salicylic acid (138.12 g/mol) gives 0.01448 mol, i.e. 2.61 g of aspirin.
How do I find the limiting reagent?
Divide the moles of each reactant by its coefficient; the smallest value marks the limiting reagent. The calculator shows this column in its table.
What is the theoretical yield of CO₂ from burning 16 g of methane?
CH₄ + 2 O₂ → CO₂ + 2 H₂O; 16.0 g ÷ 16.04 g/mol = 0.998 mol CH₄ → 0.998 mol CO₂ × 44.01 g/mol ≈ 43.9 g (with oxygen in excess).
What is the difference between theoretical and actual yield?
Theoretical yield is calculated; actual yield is what you isolate. Their ratio × 100 is the percent yield.
Can the theoretical yield be in moles?
Yes – the calculator gives both moles and grams. Use grams to compare with a weighed product.
Does the excess reagent affect the theoretical yield?
No. Only the limiting reagent sets the yield; the excess reagent is left over (its remaining mass is listed).