Enter the coefficients and available moles of two reactants (aA + bB → cC) to find the limiting reagent, moles of product formed, and theoretical yield mass.
Stoichiometry is the arithmetic of balanced chemical equations: once a reaction is balanced, the coefficients act as a mole ratio that connects any reactant to any product. This calculator applies that ratio to two reactants at once to find which one runs out first — the limiting reactant — and how much product that limit actually allows.
For a balanced reaction aA + bB → cC, the coefficients a, b, and c give the exact mole ratio the reaction consumes and produces in. If you know how many moles of A you have, the moles of product C it could produce (if A were the only constraint) is (moles A / a) × c — and the same logic applies to B.
Compute how much product each reactant could form on its own, assuming it reacts completely. Whichever reactant produces the smaller amount of product is the limiting reactant — it runs out first and stops the reaction, no matter how much of the other reactant remains. The larger of the two is the excess reactant.
Once you know the limiting reactant, the theoretical yield in moles is simply whichever "product from X" value is smaller. Converting to mass just requires multiplying by the product's molar mass: theoretical yield (g) = limiting moles of product × molar mass (g/mol). This is the maximum mass of product the reaction can form — real (actual) yield is always equal to or less than this, and percent yield = actual/theoretical × 100%.
Consider 2H₂ + O₂ → 2H₂O (a = 2, b = 1, c = 2), with 5 mol H₂ and 2 mol O₂ available, and water's molar mass 18.02 g/mol. Product from H₂: (5/2)×2 = 5 mol. Product from O₂: (2/1)×2 = 4 mol. O₂ gives the smaller value, so O₂ is the limiting reactant, and the theoretical yield is 4 mol × 18.02 g/mol = 72.08 g of water. H₂ is in excess — checking how much H₂ the limiting O₂ actually consumes, (2/1)×2 = 4 mol H₂ used, leaving 1 mol H₂ unreacted.
That means the reactants are present in exactly the stoichiometric ratio the equation calls for — neither is in excess, both are fully consumed, and the calculator will show zero excess for both. This is the ideal case for maximizing atom economy and minimizing waste reactant.
In industrial and lab settings, reactants often have very different costs, so a process is often deliberately run with one reactant in excess (usually the cheaper one) to ensure the more expensive or reaction-limiting one is fully consumed, maximizing its conversion to product.
Theoretical yield is the maximum mass of product predicted purely from stoichiometry, assuming the reaction goes to completion with no losses. Actual yield is what you measure in a real experiment or process, which is almost always lower due to side reactions, incomplete conversion, or product lost during recovery. Percent yield = (actual yield / theoretical yield) × 100%.
This tool checks two reactants directly. For reactions with three or more reactants, apply the same logic pairwise — compute the product each reactant could form alone, then take the overall minimum across all of them; that reactant is the true limiting one.