Stoichiometry Calculator

Stoichiometry: The Math of Chemical Reactions

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It lets you predict how much product will form, or how much reactant you need, for any amount you start with.

The Mole Ratio

The coefficients in a balanced chemical equation are the mole ratios. They tell you exactly how many moles of each substance react or are produced relative to each other.

Example: Combustion of hydrogen 2 H₂ + O₂ → 2 H₂O

Mole ratios:

• 2 mol H₂ : 1 mol O₂ : 2 mol H₂O
• To produce 4 mol water, you need 4 mol H₂ and 2 mol O₂.

The Three-Step Method

1. Convert the given quantity to moles (using molar mass, if you were given grams).
2. Apply the mole ratio from the balanced equation.
3. Convert back to the desired unit (moles, grams, or liters for gases).

Step 1: moles = mass ÷ molar mass

Step 2: moles of product = moles of reactant × (coefficient of product ÷ coefficient of reactant)

Step 3: mass = moles × molar mass

The calculator above does all three. Set the first dropdown to Grams and it will run step 1 for you, which is how most textbook problems are actually worded: you are handed a mass on a balance, not a mole count.

Common Molar Masses

A Worked Example

Burning hydrogen: 2 H₂ + O₂ → 2 H₂O. You have 8.00 g of H₂ and want to know the mass of water.

• Step 1: 8.00 g ÷ 2.016 g/mol = 3.9683 mol H₂
• Step 2: mole ratio H₂O:H₂ is 2:2 = 1.0000, so 3.9683 mol of water
• Step 3: 3.9683 mol × 18.015 g/mol = 71.488 g of water

Set the dropdown to Grams, then enter 8 for the amount, 2.016 for the given molar mass, 2 and 2 for the coefficients, and 18.015 for the target molar mass. You should get exactly that.

Percent Yield

In real experiments, reactions rarely go to completion:

Percent yield = (Actual yield ÷ Theoretical yield) × 100%

Typical lab reactions yield 70% to 95%. A yield below 50% usually points at technique or reaction conditions rather than at the chemistry.

Limiting Reagent

When two or more reactants are present, the one that runs out first is the limiting reagent. It sets a ceiling on how much product can form. The other reactant is in excess, so some of it is left over when the reaction stops.

This calculator assumes you already know which reactant is limiting and are working from that one. If you do not, work it out first, because running these numbers against a reactant that was in excess is the single most common way a stoichiometry answer comes out too large.