Limiting Reagent Calculator

In a reaction, one reactant usually runs out before the others. That limiting reagent decides how much product can form. Type a balanced chemical equation and the mass of each reactant, and this calculator finds the limiting reagent, the theoretical yield of every product in grams and moles, and how much of each excess reactant is left over. Add the mass of product you actually collected and it gives the percent yield.

It reads formulas with parentheses, brackets and hydrates, works out molar masses from IUPAC atomic weights, and checks that your equation is balanced before using it. It handles up to three reactants and any number of products, and the chart shows which reactant runs out first.

moles = mass ÷ molar mass

limiting reagent = smallest (moles ÷ coefficient)

product moles = (limiting moles ÷ its coefficient) × product coefficient

percent yield = actual ÷ theoretical × 100

Dividing each reactant's moles by its coefficient in the balanced equation puts them on the same footing: it is how many times the reaction, as written, could run on that reactant alone. The smallest value wins, and every product is calculated from it. Comparing the larger reactants with this value shows how much of each is consumed and how much remains.

Burning hydrogen. 10 g of H₂ and 64 g of O₂: oxygen limits and 72.06 g of water forms, with 1.94 g of hydrogen left over.

Ammonia. 28 g of N₂ with 9 g of H₂: N₂ limits, giving 34.04 g of NH₃.

Iron from ore. 1 kg of Fe₂O₃ with 600 g of CO: Fe₂O₃ is limiting and the theoretical yield is 699.4 g of iron. Collecting 600 g is a 85.8% yield.

Aspirin. 2.0 g of salicylic acid with 5.0 g of acetic anhydride gives a theoretical 2.61 g of aspirin; a student collecting 2.1 g has a 80.5% yield.

• Comparing grams instead of moles. The reactant with less mass is not necessarily limiting; 10 g of hydrogen is far more moles than 64 g of oxygen.
• Ignoring coefficients. In N₂ + 3H₂, hydrogen is used three times as fast, so it must be compared after dividing by 3.
• Using an unbalanced equation. The coefficients drive the whole calculation; the calculator refuses unbalanced equations for this reason.
• Percent yield above 100%. This usually means the product is wet or contains impurities, not that extra product formed.

In practice, chemists often add more of the cheaper or easier-to-remove reactant so that the expensive one is fully used. In aspirin synthesis, acetic anhydride is used in excess because it is cheap and the leftover is easily destroyed with water. In the blast furnace, extra carbon monoxide drives the reduction of iron ore toward completion. Choosing which reactant should limit is part of designing a reaction, and this calculator shows exactly how much excess you are carrying.

Write element symbols with the correct capital and lowercase letters, such as Co for cobalt and CO for carbon monoxide. Use parentheses for groups, as in Ca(OH)2 or Al2(SO4)3, and a dot or asterisk for water of hydration, as in CuSO4·5H2O. Coefficients go before each formula, and state symbols such as (aq) or (s) are ignored. Separate the two sides with an arrow written as -> or with an equals sign.