βοΈ Limiting Reagent Calculator
Enter reactant masses, molar masses, and coefficients to find the limiting reagent.
For reaction: aΒ·A + bΒ·B β Products
Reactant A
Reactant B
How to Use This Calculator
Enter the mass, molar mass, and stoichiometric coefficient for each of the two reactants. The calculator converts each to moles, divides by the coefficient, and identifies the reactant with the smaller result as the limiting reagent.
Write out your balanced equation first. For 2Hβ + Oβ β 2HβO, the coefficients are 2 (for Hβ) and 1 (for Oβ).
Enter the mass of reactant A in grams and its molar mass. For 10 g of Hβ with molar mass 2.016 g/mol, type 10 and 2.016. Set the coefficient to 2.
Do the same for reactant B. For 80 g of Oβ with molar mass 32.00 g/mol and coefficient 1, type those values on the right.
Click Find Limiting Reagent. The result shows both mole ratios, names the limiting reagent, and tells you how many grams of the excess reactant remain.
Limiting Reagent Formula
Dividing each reactant's moles by its stoichiometric coefficient gives the mole ratio, which is a common currency for comparing reactants. The reactant with the smaller ratio runs out first and stops the reaction. The other reactant is in excess, meaning some of it remains unreacted after the limiting reagent is fully consumed.
Worked Examples
Where This Calculation Comes Up
Identifying the limiting reagent is a required step in almost every quantitative chemistry problem that involves two reactants. Before you can calculate how much product forms (theoretical yield) or how much excess reagent remains, you must find the limiting reagent first. This comes up in synthesis lab reports, where you weigh out your starting materials and need to know which one controls how much product you can collect.
In industrial chemical production, deliberately controlling the limiting reagent is an important strategy. Running a reaction with an excess of a cheap reagent ensures that the expensive one is fully consumed, avoiding waste. In pharmaceutical manufacturing, knowing the limiting reagent and theoretical yield lets quality control teams calculate the percent yield and decide whether a batch meets specification. Even in cooking, making scrambled eggs is a limiting reagent problem: if you have 3 eggs and only enough butter for 2 servings, the butter is your limiting reagent.
Frequently Asked Questions
What is a limiting reagent?
The limiting reagent (or reactant) is the substance that is completely consumed in a reaction, limiting the amount of product that can form.
How do you find the limiting reagent?
Convert each reactant mass to moles, divide by the stoichiometric coefficient, and the reactant with the smallest value is the limiting reagent.
What is the excess reagent?
The excess reagent is the reactant that is left over after the limiting reagent is completely consumed.
How does limiting reagent affect theoretical yield?
Theoretical yield is calculated from the moles of the limiting reagent using the stoichiometric ratio to the product.
Example: 10g Hβ reacts with 80g Oβ to form water (2Hβ + Oβ β 2HβO)?
Hβ: 10/2.016 = 4.96 mol Γ· 2 = 2.48; Oβ: 80/32 = 2.5 mol Γ· 1 = 2.5. Hβ is the limiting reagent.