Limiting reactant calculator

Why the smaller mass is not always the limiting reactant

A balanced equation compares amounts in moles, with different proportions for different reactants. First convert each mass using its own molar mass, then divide each mole amount by its coefficient. The smaller quotient sets how far the written reaction can proceed.

The grams to moles calculator is useful when you only need the first step for one substance. Here, the extra comparison connects two reactants through a supplied reaction.

Worked example: 3 mol H₂ and 1 mol O₂

For 2 H₂ + 1 O₂ → 2 H₂O, hydrogen gives 3 ÷ 2 = 1.5 and oxygen gives 1 ÷ 1 = 1. Oxygen is limiting. The theoretical product is 1 × 2 = 2 mol H₂O, or 36.03 g using this app's atomic weights. Hydrogen has 3 − 2 = 1 mol left, equivalent to 2.016 g.

The example button fills these values and labels the output. It is a stoichiometry exercise, not an instruction to carry out the reaction. Replace every formula, coefficient and available amount with information from your own problem.

Entering a reaction without changing its meaning

1. Start with an established equation. Include all products, even if you are interested in only one.
2. Keep coefficients separate from subscripts. Two H₂O units and H₂O₂ have different compositions. Changing a subscript changes the substance.
3. Enter the available amount of each reactant. A solution's total mass is not necessarily the mass of its reacting ingredient. Determine the relevant amount first.
4. Check the balance table. Equal atom totals verify the arithmetic of the written equation. They do not prove that the proposed reaction occurs.

Coefficients may be unreduced positive whole numbers up to 1,000. Scaling every coefficient by the same factor leaves product and excess amounts unchanged. To keep the input readable, this version supports exactly two distinct reactants and up to four products. It does not accept charges, electrons, state symbols or unchanged spectators.

Theoretical yield is a ceiling within a model

The reported product amounts assume complete use of the limiting reactant through the entered reaction. Actual recovery may be lower because the reaction is incomplete, other reactions occur, or material is lost. This calculator does not estimate reaction feasibility, rate, equilibrium, purity or measured yield.

If the two reactants are in exact stoichiometric proportion, both are consumed in the model. An available amount of zero is allowed and gives zero theoretical products; it is different from leaving a required field blank. A balance error stops the calculation rather than silently changing the coefficients.

Formula masses use CIAAW's 2024 abridged atomic weights. The app uses central values and does not propagate their uncertainties. Isotope-enriched materials require other data and are outside this tool's formula model.

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