Stoichiometry Calculator

Stoichiometry is the bookkeeping of chemistry: it tells you how much product a given amount of reactant can possibly make, and how much starting material you need to hit a target amount. Balances measure grams, but reactions happen in moles, so every stoichiometry problem is really a three-part trip: grams to moles, moles to moles, moles back to grams. Get that habit right and yield calculations, reagent scale-ups, and lab prep sheets all become mechanical.

How to Calculate Stoichiometry (step by step)

Step One: Convert the known mass to moles

Divide the mass of your known substance by its molar mass. This is the only step that touches the balance reading.

moles of reactant = mass in grams ÷ molar mass (g/mol)

moles of H₂ = 10 ÷ 2.016 = 4.9603 mol

Step Two: Apply the mole ratio from the balanced equation

The coefficients in the balanced equation are a mole ratio, not a mass ratio. Multiply by the product coefficient over the reactant coefficient.

moles of product = moles of reactant × (product coefficient ÷ reactant coefficient)

2 H₂ + O₂ → 2 H₂O
moles of H₂O = 4.9603 × (2 ÷ 2) = 4.9603 mol

Step Three: Convert product moles back to mass

Multiply the moles of product by the product’s molar mass. This result is the theoretical yield: the absolute maximum a perfect reaction could give you.

theoretical yield = moles of product × molar mass of product (g/mol)

mass of H₂O = 4.9603 × 18.015 = 89.36 g

Step Four: Compare with what you actually collected

Real reactions lose material to side reactions, incomplete conversion, and transfer losses. Percent yield quantifies that gap.

percent yield = (actual yield ÷ theoretical yield) × 100

percent yield = (75 ÷ 89.36) × 100 = 83.93%

What your results mean

Moles of reactant (4.9603 mol) is your known quantity translated into particle counts. Moles of product (4.9603 mol) is that count rescaled by the equation; here the ratio is 2:2, so the numbers match, but with something like N₂ + 3 H₂ → 2 NH₃ the ratio would change them noticeably.

Theoretical yield (89.36 g) is your ceiling. Notice that 10 g of hydrogen becomes roughly 89 g of water: mass is conserved because oxygen joins in, which is exactly why you can never reason about yields using grams alone.

Percent yield (83.93%) is the practical verdict. For routine inorganic reactions 90% and up is expected, 70-90% is normal for multi-step organic work, and anything under 50% usually points to a procedural problem rather than chemistry. Molecules of product (2.99 × 10²⁴) is the same amount expressed as individual particles via the Avogadro constant, useful for gas-phase and spectroscopy work.

Common mistakes to avoid

Using mass ratios instead of mole ratios is the number one error; coefficients never multiply grams directly. Second is forgetting subscripts when computing molar mass: H₂ is 2.016 g/mol, not 1.008. Third is assuming your known reactant is the limiting one when both reactants are measured out; if you weighed out two reactants, check both.

Handy molar masses

To scale a reaction in reverse, put your target product in the reactant slots and the starting material in the product slots: the same arithmetic then tells you how many grams of reagent to weigh out.