What Is Theoretical Yield?
The theoretical yield is the maximum amount of product that could be formed from a given amount of starting material, assuming the reaction is 100% efficient — meaning every molecule reacts perfectly, no side products form, and nothing is lost on glassware or equipment.
In reality, the actual yield is always less than the theoretical yield. Knowing the theoretical yield allows chemists to calculate the percent yield (actual yield ÷ theoretical yield × 100%), which measures how efficient a reaction is.
Theoretical Yield Formula
The theoretical yield is calculated in two steps:
Step 1 — Moles of limiting reagent:
nlim = mlim / (Mlim × clim)
Step 2 — Theoretical yield (mass of product):
mproduct = Mproduct × nlim × cproduct
Where:
- nlim — moles of the limiting reagent
- mlim — mass of the limiting reagent (g or oz)
- Mlim — molar mass of the limiting reagent (g/mol)
- clim — stoichiometric coefficient of the limiting reagent
- mproduct — theoretical yield (mass of product, g or oz)
- Mproduct — molar mass of the desired product (g/mol)
- cproduct — stoichiometric coefficient of the desired product
How to Calculate Theoretical Yield
- Write the balanced chemical equation for the reaction.
- Identify the limiting reagent — the reactant that will be fully consumed first. If unsure, plug in your reagents one at a time and pick the one that gives the fewest moles.
- Find the stoichiometric coefficients (the numbers in front of each formula in the balanced equation).
- Look up or calculate the molar mass of the limiting reagent and the desired product.
- Enter the values into this calculator, or apply the formula above manually.
Measurement Systems
This calculator supports both the metric system (grams) and the American/Imperial system (ounces). Molar masses are always entered in g/mol regardless of the selected system; the calculator converts internally.
| System | Input mass unit | Result unit |
|---|---|---|
| Metric | grams (g) | grams (g) |
| Imperial (American) | ounces (oz) | ounces (oz) |
Conversion used: 1 oz = 28.3495 g
Example: Theoretical Yield Calculation
Consider the synthesis of potassium chloride (KCl) from sodium chloride (NaCl) and potassium nitrate (KNO₃):
NaCl + KNO₃ → KCl + NaNO₃
Suppose you start with 10 g of NaCl as the limiting reagent:
- Molar mass of NaCl (Mlim) = 58.44 g/mol
- Stoichiometry of NaCl (clim) = 1
- Molar mass of KCl (Mproduct) = 74.55 g/mol
- Stoichiometry of KCl (cproduct) = 1
nlim = 10 / (58.44 × 1) = 0.1711 mol mKCl = 74.55 × 0.1711 × 1 ≈ 12.76 g
The theoretical yield is ≈ 12.76 g of KCl.
Frequently Asked Questions
What is the limiting reagent?
The limiting reagent is the reactant that is completely consumed during the reaction, thereby limiting the amount of product that can form. The theoretical yield is always calculated based on the limiting reagent. If you are unsure which reagent is limiting, calculate the moles for each reagent separately and choose the one that gives the smallest number of moles.
Why can't actual yield exceed theoretical yield?
The theoretical yield assumes 100% conversion with no losses. In practice, reactions never achieve perfect efficiency — some product is lost due to side reactions, incomplete reactions, or handling losses. Therefore the actual yield is always ≤ theoretical yield.
What is stoichiometry?
Stoichiometry refers to the numerical coefficients in a balanced chemical equation. For example, in 2 H₂ + O₂ → 2 H₂O, the stoichiometry of H₂ is 2, of O₂ is 1, and of H₂O is 2. These ratios are essential for calculating how much product forms from a given amount of reactant.
How do I find the molar mass?
The molar mass of a compound is the sum of the atomic masses of all atoms in its formula, expressed in g/mol. Atomic masses are found on the periodic table. For example, NaCl = 22.99 (Na) + 35.45 (Cl) = 58.44 g/mol.
Can I use this calculator to find the mass of reactant needed?
Yes — rearrange the formula: if you know the desired product mass, you can solve for mlim. Alternatively, enter a target yield as the "mass" and adjust until you reach the desired output.