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Stoichiometry Calculator

Enter a known substance and amount, the mole ratio coefficients, and the target substance to find how much product is formed.

What stoichiometry computes

A balanced chemical equation encodes mole ratios. The coefficients tell you how the reactants and products are related quantitatively, and stoichiometry is the procedure for translating “how much of A do I have?” into “how much of B will I get?”

The general path is always the same three-step conversion:

grams of A → moles of A → moles of B → grams of B

  1. Mass to moles using the molar mass of A (n = m/M).
  2. Moles of A to moles of B through the mole ratio from the balanced equation.
  3. Moles of B back to mass (or volume, for a gas at STP) using the molar mass of B.

The middle step is where the balanced equation does its work; the outer steps are unit conversions. The reason you cannot shortcut directly from grams to grams is that the ratio is in moles, and one mole of one substance does not weigh the same as one mole of another.

Inputs

  1. A balanced equation (e.g., 2H2 + O2 -> 2H2O).
  2. The known substance and its amount, in grams, moles, or liters of gas at STP.
  3. The target substance.

The calculator parses the equation, applies the three-step path, and shows each conversion explicitly.

Worked examples

Mass to mass. How many grams of water form when 4.0 g H₂ reacts with excess O₂? Equation: 2H₂ + O₂ → 2H₂O. Moles of H₂ = 4.0 / 2.016 = 1.984. H₂:H₂O = 2:2 = 1:1, so moles of H₂O = 1.984. Mass = 1.984 × 18.015 = 35.7 g.

Mass to gas volume at STP. How many liters of CO₂ at STP from 12.0 g of carbon? C + O₂ → CO₂. Moles of C = 12.0 / 12.011 = 0.999. C:CO₂ = 1:1. Volume at STP = 0.999 × 22.414 = 22.4 L.

Moles to mass. 0.500 mol NaOH + HCl → NaCl + H₂O. NaOH:NaCl = 1:1, so 0.500 mol NaCl. Mass = 0.500 × 58.44 = 29.2 g.

Methane combustion. Grams of O₂ needed to burn 32.0 g CH₄? CH₄ + 2O₂ → CO₂ + 2H₂O. Moles of CH₄ = 32.0 / 16.04 = 1.995. CH₄:O₂ = 1:2, so moles of O₂ = 3.990. Mass = 3.990 × 32.00 = 127.7 g.

Where stoichiometry lives

  • Predicting reaction yields before running a synthesis.
  • Scaling a literature procedure up or down by an arbitrary factor.
  • Calculating raw material requirements for an industrial run.
  • Finding emission masses from combustion equations.
  • Drug synthesis, where the molar ratios determine reagent quantities and excess calculations.

Pair this with the Limiting Reagent Calculator when you have specific amounts of two or more reactants and need to find which one runs out first.

Frequently Asked Questions

What is stoichiometry?
Stoichiometry is the quantitative relationship between substances in a balanced chemical equation. The coefficients in the equation are mole ratios, and those ratios let you calculate how much product forms from a given amount of reactant — or how much reactant you need to get a target amount of product. It is the bridge from a balanced equation to actual amounts.
How do you convert grams to moles?
Divide grams by molar mass: n = m / M. 36 g of water with M = 18.015 g/mol gives 36/18.015 = 2.00 mol. The molar mass comes from summing atomic masses according to the formula. The reverse direction multiplies: m = n × M.
What is a mole ratio?
A mole ratio is the ratio of stoichiometric coefficients between two substances in a balanced equation. In 2H2 + O2 → 2H2O the ratio of H2 to O2 is 2:1, of H2 to H2O is 2:2 = 1:1, and of O2 to H2O is 1:2. Mole ratios only work in moles — applying them to grams directly gives wrong answers because different substances have different molar masses.
How do you do a mass-to-mass stoichiometry problem?
Three steps: convert grams of the known substance to moles using its molar mass, multiply by the mole ratio from the balanced equation to get moles of the target, then multiply by the target's molar mass to get grams. Mole ratio only acts on moles, so the conversion-ratio-conversion sandwich is unavoidable. Skipping to grams×ratio is the most common error.
What does STP mean for gas stoichiometry?
STP is Standard Temperature and Pressure: 273.15 K (0 °C) and 1 atm. At STP one mole of an ideal gas occupies 22.414 L — the molar volume — which lets you swap between moles and liters for gases without using PV = nRT. Note that IUPAC redefined STP in 1982 to use 100 kPa, giving a molar volume of 22.711 L; most US textbooks still teach the 1 atm version.
What is theoretical yield?
Theoretical yield is the amount of product you would get if the reaction proceeded to completion with no losses, calculated from the limiting reagent through the mole ratio. Actual yield is what you measure after the reaction; percent yield is (actual / theoretical) × 100. Yields below 100% are normal and reflect side reactions, incomplete conversion, and recovery losses.