Calculate the moles and mass of a product or reactant in a balanced chemical reaction, using the mole ratio between two substances.
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Reaction stoichiometry is the use of the coefficients from a balanced chemical equation to determine the relative amounts of reactants and products in a given reaction. This calculator relies on the principle that the ratio of moles of two substances in a balanced reaction equals the ratio of their coefficients in the equation, i.e., n(B) = n(A) × (coefficient of B ÷ coefficient of A). To find the number of moles of the first substance A, you can enter it directly if known, or calculate it from its mass and molar mass (n = mass ÷ M) if only the mass is available. After finding the number of moles of the target substance B, it's automatically converted to mass in grams by multiplying by its molar mass. Illustrative example: in the hydrogen combustion reaction 2H₂ + O₂ → 2H₂O, if 4 moles of H₂ (coefficient 2) react, the resulting water H₂O (also coefficient 2) equals 4 moles as well, because the ratio between the two coefficients is 2:2 = 1:1. This principle is used to determine the required amounts of reactants in industry and laboratories, and to calculate the theoretical yield of any chemical reaction.
A balanced chemical equation is often described as a recipe, and that comparison holds up well: just as a cake recipe specifies exact ratios of flour to sugar to eggs, a balanced equation specifies exact mole ratios between every reactant and product, and stoichiometry is simply the practical skill of using those ratios to scale a reaction up or down to whatever actual quantity you're working with.
The coefficients in a balanced equation aren't arbitrary numbers — they represent the exact whole-number ratio of moles in which substances react and form, a ratio that must hold true regardless of whether you're running a reaction with a few molecules in a theoretical calculation or scaling it up to industrial quantities involving tons of material. This scale-independence is exactly why mole ratios, not mass ratios, are the correct tool for stoichiometry calculations.
The reason moles rather than mass must be used directly comes from what a balanced equation actually describes: chemical reactions occur between individual atoms and molecules in fixed whole-number ratios, and moles are the practical unit for counting those particles. Mass ratios between reactants and products aren't fixed the same simple way, since different substances have different molar masses — this is exactly why any stoichiometry calculation that starts with a known mass must first convert that mass to moles before applying the equation's coefficient ratios, then convert the resulting moles of the target substance back to mass if a mass answer is needed.
This calculation underlies genuinely practical questions in both industry and the laboratory: how much of a raw material needs to be purchased to produce a target quantity of product, how much of a limiting reagent is available compared to what a reaction actually requires, and what the theoretical maximum yield of a reaction should be — a benchmark against which actual, real-world yield (always somewhat lower due to side reactions, incomplete reactions, and material losses) gets compared to calculate a reaction's efficiency.
Industrial chemical processes rely on stoichiometric calculations at genuinely enormous scale, where getting the reactant ratios even slightly wrong translates into real, significant material and cost waste — a chemical plant designed to produce a specific compound needs precise stoichiometric planning to determine exactly how much of each raw material to purchase and feed into the process to achieve the target production volume without excess unreacted material left over.
You need to balance your chemical equation first (so that atoms of each element are equal on both sides) — the coefficients are the numbers in front of each formula in that balanced equation.
Enter the mass and molar mass of substance A instead of moles directly — the calculator will convert mass to moles automatically using n = mass ÷ molar mass before applying the mole ratio.
No — you need to supply the coefficients from an already-balanced equation; this tool only applies the mole-ratio relationship between two substances once the balanced equation is known.