Determine which reactant will run out first in your chemical reaction.
Reactant A
Reactant B
How to Calculate Limiting Reagent: A Complete Guide
In stoichiometry, the limiting reagent (or limiting reactant) is the substance that is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent, since the reaction cannot continue without it.
The 3-Step Calculation Method
To find the limiting reagent manually, follow these steps:
Calculate Moles: Convert the mass of each reactant to moles using the formula: Moles = Mass / Molar Mass.
Find the Molar Ratio: Divide the number of moles of each reactant by its stoichiometric coefficient from the balanced chemical equation.
Compare: The reactant with the smallest resulting ratio is your limiting reagent.
Example Calculation
Consider the reaction: 2H₂ + O₂ → 2H₂O
Reactant
Mass Provided
Molar Mass
Moles
Coefficient
Ratio (Moles/Coeff)
Hydrogen (H₂)
4.0 g
2.02 g/mol
1.98 mol
2
0.99
Oxygen (O₂)
10.0 g
32.00 g/mol
0.31 mol
1
0.31
In this example, Oxygen has the lower ratio (0.31 vs 0.99), making Oxygen the limiting reagent.
Why is this important?
Identifying the limiting reagent is crucial for calculating the theoretical yield. The amount of product created is always based on the moles of the limiting reagent, not the excess reagent. This helps chemists minimize waste and maximize efficiency in laboratory and industrial settings.
function calculateLimitingReagent() {
var massA = parseFloat(document.getElementById('massA').value);
var molarA = parseFloat(document.getElementById('molarA').value);
var coeffA = parseFloat(document.getElementById('coeffA').value);
var massB = parseFloat(document.getElementById('massB').value);
var molarB = parseFloat(document.getElementById('molarB').value);
var coeffB = parseFloat(document.getElementById('coeffB').value);
var resultDiv = document.getElementById('lim-calc-result');
// Validation
if (isNaN(massA) || isNaN(molarA) || isNaN(coeffA) || isNaN(massB) || isNaN(molarB) || isNaN(coeffB)) {
resultDiv.style.display = 'block';
resultDiv.className = 'res-error';
resultDiv.innerHTML = 'Error: Please enter valid numbers for all fields.';
return;
}
if (molarA <= 0 || molarB <= 0 || coeffA <= 0 || coeffB <= 0) {
resultDiv.style.display = 'block';
resultDiv.className = 'res-error';
resultDiv.innerHTML = 'Error: Molar mass and coefficients must be greater than zero.';
return;
}
// Step 1: Calculate Moles
var molesA = massA / molarA;
var molesB = massB / molarB;
// Step 2: Calculate Stoichiometric Ratios
var ratioA = molesA / coeffA;
var ratioB = molesB / coeffB;
// Step 3: Compare
var limitingReagent = "";
var excessReagent = "";
if (ratioA < ratioB) {
limitingReagent = "Reactant A";
excessReagent = "Reactant B";
} else if (ratioB < ratioA) {
limitingReagent = "Reactant B";
excessReagent = "Reactant A";
} else {
limitingReagent = "Both (Stoichiometric Proportion)";
}
// Output results
resultDiv.style.display = 'block';
resultDiv.className = 'res-success';
var html = "