Stoichiometry Calculator
Convert the mass of one substance in a balanced equation to the mass of another. Enter masses, molar masses and coefficients and see every step.
What this calculator does
Give the mass of reactant A, the molar masses and the coefficients from the balanced equation, and the calculator returns the mass of B with the three conversion steps shown.
When you need it
Stoichiometry answers how much product a reaction makes or how much reactant you need. It is used for lab yields, for scaling a synthesis up or down, and in nearly every general chemistry exam.
Formulas
- n(A) = m(A) ÷ M(A)
- n(B) = n(A) × (coefficient of B ÷ coefficient of A)
- m(B) = n(B) × M(B)
How to use it
- Balance the equation first and read the coefficients a and b.
- Enter the mass of A, the molar mass of A and the molar mass of B.
- Enter the coefficients of A and B.
- Press Enter to get the mass of B and the three steps: grams to moles, mole ratio, moles to grams.
Worked example
In 2 H2 + O2 → 2 H2O, how many grams of water form from 4.00 g of H2?
- n(H2) = 4.00 g ÷ 2.016 g/mol = 1.984 mol
- Mole ratio H2O : H2 = 2 : 2, so n(H2O) = 1.984 mol
- m(H2O) = 1.984 mol × 18.015 g/mol = 35.74 g
Answer
4.00 g of H2 gives 35.7 g of H2O.
Common mistakes
- Using the equation before it is balanced, so the mole ratio is wrong.
- Using the ratio of masses instead of the ratio of moles.
- Writing the coefficients the wrong way round (b ÷ a, not a ÷ b).
Related tools
Frequently asked questions
What is stoichiometry?
Stoichiometry is the calculation of amounts of reactants and products in a chemical reaction, using the mole ratios from the balanced equation.
What are the steps of a stoichiometry problem?
Convert grams to moles, use the mole ratio from the balanced equation, then convert moles of the answer back to grams.
What if one reactant runs out first?
Then the amount of product is set by the limiting reagent. Use the limiting reagent calculator to find it.