Stoichiometry Calculator
Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction, derived from the balanced equation. Every balanced equation has coefficients that tell you how many moles of each substance participate: these coefficients define the mole ratio that links any two substances in the reaction. A mass-to-mass stoichiometry calculation converts the mass of a known substance (a reactant you are starting with, or a product you have measured) into the mass of any other substance in the equation. The four steps are always the same: convert the given mass to moles using the molar mass of that substance; multiply by the mole ratio from the balanced equation; multiply by the molar mass of the wanted substance to get the predicted mass. This calculator carries out all four steps. You enter the mass of the given substance, its molar mass, the mole ratio (moles of wanted substance per mole of given substance, from the balanced equation), and the molar mass of the wanted substance. The mole ratio field accepts any positive decimal. For example, in 2H2 + O2 gives 2H2O: to find moles of O2 from moles of H2, the ratio is 1/2 = 0.5; to find moles of H2O from moles of H2, the ratio is 2/2 = 1. The worked example uses the hydrogen-oxygen-water reaction. The calculator is used in chemistry at all levels, from secondary school through to university and industry. Results are the theoretical values and do not account for incomplete reactions or yield losses.
The mole ratio comes from the coefficients in the balanced equation: coefficient of wanted divided by coefficient of given. For 2H2 + O2 gives 2H2O, the ratio of O2 to H2 is 1/2 = 0.5.
How it works
Mass-to-mass stoichiometry applies the roadmap: mass_given / MM_given x ratio x MM_wanted = mass_wanted. Step 1: moles_given = mass_given / MM_given. Step 2: moles_wanted = moles_given x ratio. Step 3: mass_wanted = moles_wanted x MM_wanted. The mole ratio is taken from the coefficients in the balanced equation and can be any positive decimal. For a 2:3 ratio (2 mol given, 3 mol wanted), enter 1.5.
Worked example
In the reaction 2H2 + O2 gives 2H2O, you start with 2 g of H2 (molar mass 2 g/mol). Step 1: moles H2 = 2 / 2 = 1.0000 mol. The mole ratio of O2 to H2 is 1/2 = 0.5. Step 2: moles O2 = 1.0000 x 0.5 = 0.5000 mol. Step 3: mass O2 = 0.5000 x 32 = 16.00 g of O2 is needed. These match the default values pre-filled above. (Note: if the wanted substance were H2O at 18 g/mol with ratio 1, then 1.0000 mol x 18 = 18 g H2O.)
Related calculators
- Theoretical Yield Calculator: find the maximum product mass from the limiting reagent and mole ratio.
- Percent Yield Calculator: compare actual product obtained with the theoretical maximum.
- Molar Mass Calculator: find the molar mass of any reactant or product from its chemical formula.
- Percent Error Calculator: measure the deviation of an experimental mass from the stoichiometric prediction.