Identify which reactant limits product formation and calculate excess reagent amounts
1A + 1B → Products
General balanced equation
Unit: mol
Unit: mol
The limiting reagent calculator determines which reactant in a chemical reaction will be completely consumed first, thereby limiting the amount of product that can be formed. It compares the mole ratios of available reactants to their stoichiometric coefficients and calculates how much excess reagent remains unreacted.
Compare: moles available / stoichiometric coefficient
Smallest ratio = Limiting Reagent
Step 1
Write the balanced chemical equation
Step 2
Divide available moles by stoichiometric coefficient for each reactant
Step 3
The reactant with the smallest ratio is the limiting reagent
Step 4
Calculate excess using: Excess = Initial moles - Moles consumed
Example Reaction:
2H₂ + O₂ → 2H₂O
If you have 3 mol H₂ and 2 mol O₂: H₂ ratio = 3/2 = 1.5, O₂ ratio = 2/1 = 2.0 → H₂ is limiting
Given Reaction
2H₂ + O₂ → 2H₂O
Available: 3.0 mol H₂ and 5.0 mol O₂
Calculate Ratios
H₂: 3.0 mol / 2 = 1.5
O₂: 5.0 mol / 1 = 5.0
Identify Limiting Reagent
1.5 < 5.0, so H₂ is the limiting reagent
Calculate Excess O₂
O₂ needed = (3.0 mol H₂ / 2) × 1 = 1.5 mol
Excess O₂ = 5.0 - 1.5 = 3.5 mol remaining
Using unbalanced equations
Always balance the equation before calculations
Comparing moles directly
Must divide by stoichiometric coefficients first
Forgetting unit conversions
Convert grams to moles before calculating
Choosing largest ratio
Smallest ratio is limiting, not largest
Calculate reaction efficiency
Calculate maximum product formation
Determine empirical formula from composition
A limiting reagent (or limiting reactant) is the reactant in a chemical reaction that is completely consumed first, thereby limiting the amount of product that can be formed. Once it's used up, the reaction stops even if other reactants remain.
The limiting reagent determines the theoretical yield (maximum amount of product possible). It's crucial for industrial processes to optimize costs, minimize waste, and predict production quantities accurately.
An excess reagent is any reactant that remains after the limiting reagent is completely consumed and the reaction stops. It's present in greater quantity than needed for complete reaction with the limiting reagent.
Yes, if reactants are present in exact stoichiometric proportions (according to the balanced equation), all reactants are consumed simultaneously with no excess. This is rare in practice.
Use the formula: moles = mass (g) / molar mass (g/mol). First calculate the molar mass from the periodic table, then divide the given mass by the molar mass to get moles.
The same principle applies: calculate the moles/coefficient ratio for each reactant. The one with the smallest ratio is the limiting reagent, regardless of how many reactants are present.
The theoretical yield is always calculated based on the limiting reagent, not any excess reagents. Use the moles of limiting reagent and stoichiometry to determine the maximum product formation.
Using one reagent in excess ensures the other reagent reacts completely, maximizing product yield. It's especially useful when one reactant is expensive or hard to obtain - you use the other in excess.
Education
Stoichiometry courses and lab calculations
Laboratory
Planning experiments and predicting yields
Industry
Manufacturing optimization and cost control
Research
Synthetic chemistry and process development