Enter details for two reactants and the desired product. Provide either mass (g) and molar mass, or enter moles directly.
In most chemical reactions, reactants are not present in the exact stoichiometric ratio. The limiting reagent (also called the limiting reactant) is the substance that is completely consumed first, stopping the reaction.
The other reactant(s) remain partially unused — they are said to be in excess. Once the limiting reagent is exhausted, no more product can form regardless of how much excess reagent remains.
In industrial chemistry, identifying the limiting reagent is crucial for cost efficiency. The more expensive or hazardous reagent is often made the limiting reagent, and the cheaper one is provided in excess to ensure complete conversion. This maximises the yield of the expensive reactant.
If ratio A/coeff_A < ratio B/coeff_B, then A is limiting. If equal, both are used up exactly (stoichiometric amounts).
Balanced equation: Fe + S → FeS (ratio 1:1:1)
Step 1: Calculate moles
n(Fe) = 10 / 56 = 0.1786 mol | n(S) = 8 / 32 = 0.2500 mol
Step 2: Divide by stoichiometric coefficients (both = 1)
Fe: 0.1786/1 = 0.1786 | S: 0.2500/1 = 0.2500
Step 3: Smallest value → Fe is the LIMITING REAGENT
Step 4: Theoretical yield of FeS
n(FeS) = n(Fe) × (coeff FeS / coeff Fe) = 0.1786 × 1 = 0.1786 mol
Mass(FeS) = 0.1786 × 88 = 15.71 g
Step 5: Excess S remaining
S used = 0.1786 mol (same as Fe). S remaining = 0.2500 − 0.1786 = 0.0714 mol = 0.0714 × 32 = 2.29 g excess S
Percentage yield measures the efficiency of a reaction in practice. A yield of 100% would mean no losses — this is theoretically impossible in practice.
Theoretical yield = 15.71 g FeS. Actual yield = 12.6 g.
% yield = (12.6/15.71) × 100 = 80.2%
Atom economy is a measure of how efficiently atoms from the reactants are incorporated into the desired product. It is a property of the reaction equation, not the experimental yield.
| Reaction Type | Atom Economy | Example |
|---|---|---|
| Addition | 100% | A + B → AB |
| Rearrangement | 100% | Isomerisation |
| Substitution | <100% | Halogenation |
| Elimination | <100% | Dehydration |
High atom economy reactions are preferred in the chemical industry as they generate less waste and are more sustainable.
The limiting reagent (or limiting reactant) is the reactant that is completely consumed first in a chemical reaction, thereby stopping the reaction and determining the maximum amount of product that can be formed. The other reactant(s) are said to be in excess. The amount of product formed is determined solely by the moles of limiting reagent available.
To find the limiting reagent: (1) Convert all reactant masses to moles using moles = mass/Mr. (2) Divide each number of moles by its stoichiometric coefficient from the balanced equation. (3) The reactant with the smallest resulting value is the limiting reagent, as it has the least moles available relative to what is required by the equation.
Theoretical yield is the maximum mass of product that could be formed from a given amount of limiting reagent, assuming the reaction goes to completion with 100% efficiency. It is calculated from the moles of limiting reagent, the stoichiometric ratio to the product, and the molar mass of the product. In practice, actual yield is always less.
Percentage yield = (actual yield / theoretical yield) × 100%. For example, if the theoretical yield is 15.71 g and you obtain 12.6 g in the laboratory, the percentage yield = (12.6/15.71) × 100 = 80.2%. A percentage yield of 80% or above is generally considered good for a laboratory reaction.
Atom economy = (molar mass of desired product / total molar mass of all products) × 100%. It measures how efficiently atoms are used in a reaction. Addition reactions have 100% atom economy because all atoms in the reactants become part of the product. Reactions producing multiple products have lower atom economy. High atom economy is a key principle of green chemistry.
Actual yield is less than theoretical because: the reaction may not go to completion (especially in reversible equilibrium reactions); side reactions produce unwanted products; product is physically lost during filtration, distillation, crystallisation, or transfer between vessels; and some product may remain dissolved in the reaction solvent. Practical skill and optimised conditions help maximise actual yield.
Percentage yield measures how much of the theoretical amount of product was actually obtained — it depends on experimental conditions and technique and can vary between experiments. Atom economy is a fixed property of the balanced equation, measuring what fraction of total reactant atoms end up in the desired product. A reaction can have high atom economy but low percentage yield (or vice versa). Both matter for industrial sustainability.