Stoichiometry Calculator
Convert an amount of one substance into the amount of another using a balanced equation. Enter what you have, the two coefficients, and the molar masses.
What stoichiometry is doing
Stoichiometry is the bookkeeping of a chemical reaction. A balanced chemical equation tells you the ratio in which substances react, and stoichiometry turns that ratio into real quantities you can weigh out.
The key idea is that coefficients count particles, not grams. In this reaction:
2 H2 + O2 → 2 H2O
the 2 and the 1 and the 2 mean two molecules of hydrogen react with one of oxygen to give two of water. They do not mean 2 grams, 1 gram, and 2 grams. Grams do not work in ratios like this, because different substances have different masses per particle. Moles do, which is why every stoichiometry problem passes through moles.
The three-step road map
Nearly every mass-to-mass problem follows the same path.
- Grams of what you have → moles. Divide by its molar mass.
- Moles of what you have → moles of what you want. Multiply by the mole ratio from the balanced equation, which is the coefficient of what you want divided by the coefficient of what you have.
- Moles of what you want → grams. Multiply by its molar mass.
You cannot skip the middle step, and you cannot jump straight from grams to grams. If a problem gives you moles already, start at step two.
Worked example
How many grams of water form when 10.0 g of hydrogen burns completely in oxygen?
Step 1. Grams of hydrogen to moles. H2 has a molar mass of 2.016 g/mol.
n = 10.0 g ÷ 2.016 g/mol = 4.960 mol H2
Step 2. Apply the mole ratio. The equation is 2 H2 + O2 → 2 H2O, so the ratio of water to hydrogen is 2 to 2, which is 1.
n = 4.960 mol × (2 ÷ 2) = 4.960 mol H2O
Step 3. Moles of water to grams. Water is 18.015 g/mol.
m = 4.960 mol × 18.015 g/mol = 89.4 g of water
Notice the mass grew from 10.0 g to 89.4 g. Nothing was created, because the oxygen that joined in brought its own mass with it. Adding the 79.4 g of oxygen consumed to the 10.0 g of hydrogen gives exactly 89.4 g.
Limiting reactant
Real reactions rarely start with a perfect ratio. Whichever reactant runs out first stops the reaction, and it is called the limiting reactant. Everything else is in excess and some is left over.
To find it, do not just compare masses or even moles. Compare how many "batches" of the reaction each reactant could supply:
- Convert each reactant to moles.
- Divide each result by that reactant's coefficient.
- The smallest answer is the limiting reactant.
Suppose you have 4.0 mol H2 and 1.0 mol O2. Dividing by the coefficients gives 4.0 ÷ 2 = 2.0 for hydrogen and 1.0 ÷ 1 = 1.0 for oxygen. Oxygen is limiting, even though there is less hydrogen by mass. Base the whole calculation on the limiting reactant.
Theoretical yield, actual yield, percent yield
The mass this calculator gives you is the theoretical yield, the most you could possibly get if nothing went wrong. In practice you always collect less.
Percent yield = (actual yield ÷ theoretical yield) × 100
Product is lost when it sticks to glassware, stays dissolved in the solvent, escapes as vapor, or reacts down a side path. A percent yield above 100 is not a triumph, it is a warning sign. It usually means the product is still damp with solvent or contaminated with something else.
Mistakes worth avoiding
- Using an unbalanced equation. Balance it first. Every coefficient in the ratio depends on it.
- Treating coefficients as masses. They count particles.
- Flipping the mole ratio. The substance you want goes on top.
- Ignoring the limiting reactant. If a problem gives amounts for two reactants, it is almost certainly testing this.
- Forgetting diatomic elements. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine travel in pairs, so oxygen gas is O2 at 32.00 g/mol.
Frequently asked questions
How do I do a stoichiometry problem?
Convert the amount you know from grams into moles by dividing by its molar mass, multiply by the mole ratio from the balanced equation, then convert back to grams by multiplying by the molar mass of the substance you want.
What is a mole ratio?
A mole ratio comes straight from the coefficients of a balanced equation. In 2 H2 + O2 giving 2 H2O, the ratio of water to oxygen is 2 to 1, so every mole of oxygen produces two moles of water.
How do I find the limiting reactant?
Convert every reactant to moles, then divide each by its own coefficient in the balanced equation. The smallest result is the limiting reactant, and it sets how much product can form.
What is theoretical yield?
Theoretical yield is the maximum mass of product the balanced equation allows, assuming the reaction goes to completion and nothing is lost. It is what a stoichiometry calculation gives you.
Why can I not convert grams directly to grams?
Because the balanced equation gives a ratio of particles, not of masses. Different substances weigh different amounts per particle, so you have to pass through moles to use the ratio correctly.
Can percent yield be more than 100 percent?
Not genuinely. A figure above 100 percent normally means the product was still wet with solvent, or that impurities were weighed along with it. Dry the sample fully and weigh again.
Related terms
Next: get the g/mol figures from the molar mass calculator, or work the yield out on the percent yield calculator.