Stoichiometry Calculator
Unknown Substance (moles)
—
Unknown Substance (grams)
—
How Stoichiometry Works
Stoichiometry uses the mole ratios from a balanced chemical equation to calculate amounts of reactants and products. The coefficients in a balanced equation give the ratio: if 2A → 1B, then 2 moles of A produce 1 mole of B.
To use this calculator: enter the moles of the known substance, its coefficient in the balanced equation, and the coefficient of the unknown substance. The calculator computes moles of the unknown using: moles_unknown = moles_known × (coeff_unknown / coeff_known).
Stoichiometry is the quantitative backbone of chemistry. It's used in manufacturing to determine raw material needs, in pharmacology for drug dosing, and in environmental science for pollution calculations. Always start with a balanced equation.
Frequently Asked Questions
What is stoichiometry?
The calculation of quantities in chemical reactions using mole ratios from balanced equations. It predicts how much product forms or reactant is needed.
Why must equations be balanced?
Balanced equations obey the law of conservation of mass — atoms are neither created nor destroyed. Mole ratios only work with balanced equations.
What is a mole ratio?
The ratio of coefficients in a balanced equation. In 2H₂ + O₂ → 2H₂O, the H₂:O₂ ratio is 2:1 and H₂:H₂O ratio is 2:2 (or 1:1).
What is a limiting reagent?
The limiting reagent (or limiting reactant) is the reactant that is completely consumed first in a chemical reaction, determining the maximum amount of product that can form. To identify it, calculate how much product each reactant could produce individually using stoichiometric ratios. The reactant that produces the smaller amount of product is the limiting reagent. The other reactant(s) are in excess. For example, if you have 4 moles of H2 and 1 mole of O2 for the reaction 2H2 + O2 -> 2H2O, O2 can produce 2 moles of H2O while H2 can produce 4 moles -- so O2 is the limiting reagent and only 2 moles of H2O will form.
How do I convert between grams and moles?
To convert grams to moles, divide the mass by the molar mass: moles = grams / molar mass (g/mol). To convert moles to grams, multiply: grams = moles x molar mass. Molar mass is found by summing the atomic masses of all atoms in the molecular formula. For example, water (H2O) has a molar mass of 2(1.008) + 16.00 = 18.015 g/mol. So 36.03 grams of water = 36.03 / 18.015 = 2.0 moles. Our mole calculator handles these conversions directly.
What is percent yield and how is it calculated?
Percent yield measures the efficiency of a chemical reaction: Percent Yield = (Actual Yield / Theoretical Yield) x 100. Theoretical yield is the maximum amount of product predicted by stoichiometry, while actual yield is what you measure in the lab. A percent yield above 90% is considered excellent for most organic reactions. Real reactions rarely achieve 100% yield due to side reactions, incomplete reactions, loss during purification, and measurement errors. In industrial chemistry, percent yield directly affects profitability -- even a 1% improvement in yield for a high-volume process can save millions of dollars annually.