Stoichiometry Calculator

Frequently Asked Questions

How does stoichiometry work?

Convert the known substance to moles (mass ÷ molar mass), apply the balanced equation's mole ratio to the target, then convert back to mass if needed.

What is a limiting reagent?

The reactant that runs out first and caps the product yield. Compare each reactant's moles ÷ its coefficient - the smallest value is limiting.

Why must the equation be balanced first?

Mole ratios come directly from the balanced coefficients. An unbalanced equation gives the wrong ratio and wrong amounts.

What is percent yield?

(actual ÷ theoretical) × 100. The tool gives the theoretical yield; real reactions are lower due to side reactions and losses.

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How This Calculator Works

Stoichiometry uses the coefficients of a balanced chemical equation as a mole-ratio recipe. This calculator runs the classic three-step "mass → moles → mass" conversion. First it converts the known substance's mass to moles using moles = mass ÷ molar mass. Second, it scales by the mole ratio from the balanced equation: target moles = known moles × (target coefficient ÷ known coefficient). Third, it converts back to mass with mass = moles × molar mass. For the reaction 2H₂ + O₂ → 2H₂O, the coefficient ratio of H₂O to H₂ is 2:2 = 1:1. The optional limiting-reagent mode compares two reactants by dividing each reactant's available moles by its coefficient; whichever gives the smaller value runs out first and caps the product yield. All chemistry honours conservation of mass - atoms are only rearranged, never created or destroyed.

Worked Example: Burning Hydrogen

For 2H₂ + O₂ → 2H₂O, start with 4 g of H₂ (molar mass 2.016 g/mol). Step 1: moles of H₂ = 4 ÷ 2.016 ≈ 1.984 mol. Step 2: the H₂O-to-H₂ coefficient ratio is 2:2 = 1, so moles of H₂O = 1.984 × (2 ÷ 2) = 1.984 mol. Step 3: mass of H₂O = 1.984 × 18.015 ≈ 35.74 g. If only 32 g of O₂ (molar mass 32 g/mol = 1 mol) is available, divide each by its coefficient: H₂ gives 1.984 ÷ 2 = 0.992, O₂ gives 1 ÷ 1 = 1. H₂ has the smaller ratio, so hydrogen is the limiting reagent and oxygen is in slight excess.

Key Definitions

Mole: 6.022 × 10²³ particles (Avogadro's number) - the chemist's counting unit.

Molar mass: grams per mole, numerically equal to the formula mass in atomic mass units.

Mole ratio: the ratio of coefficients in the balanced equation - the bridge between any two species.

Limiting reagent: the reactant that runs out first, setting the maximum possible product.

Excess reagent: any reactant left over after the limiting one is consumed.

Balancing Comes First

Every stoichiometric calculation depends on a correctly balanced equation - the coefficients are the mole ratio. An unbalanced equation gives the wrong ratio and therefore the wrong answer no matter how careful the arithmetic. Balance by adjusting coefficients (never subscripts, which would change the chemical identity) until each element has equal atom counts on both sides. For example, H₂ + O₂ → H₂O is not balanced; the correct form 2H₂ + O₂ → 2H₂O has 4 H and 2 O on each side. This tool assumes you have already balanced the reaction and supplies the coefficients yourself.

Theoretical vs. Actual Yield

The mass this calculator returns is the theoretical yield - the maximum if the reaction went to completion with no losses. Real labs almost never reach it. Percent yield = (actual ÷ theoretical) × 100 captures the gap, caused by side reactions, incomplete reactions, reversible equilibria, transfer losses, and impure reactants. A 90% yield is excellent for many syntheses; multi-step routes compound losses (0.9 × 0.9 × 0.9 ≈ 73%). The calculator also assumes pure substances and exact molar masses; isotopic averages and impurities introduce small real-world deviations.

Stoichiometry Calculator

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