Stoichiometry Calculator

Enter a reaction and one amount you know. Get the moles and grams of every other reactant and product, with the mole-ratio working laid out like a lab notebook.

Separate substances with +, sides with -> or =. Coefficients are optional; the equation is balanced for you.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

O₂
O₂ needed
160g
5 mol · 32.00 g/mol
CO₂
CO₂ produced
132g
3 mol · 44.01 g/mol
H₂O
H₂O produced
72.06g
4 mol · 18.02 g/mol
  1. Moles of C₃H₈: 44.1 g ÷ 44.097 g/mol = 1 mol
  2. O₂: 1 mol × (5 ÷ 1) = 5 mol × 31.998 g/mol = 160 g
  3. CO₂: 1 mol × (3 ÷ 1) = 3 mol × 44.009 g/mol = 132 g
  4. H₂O: 1 mol × (4 ÷ 1) = 4 mol × 18.015 g/mol = 72.06 g

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The stoichiometry road map

grams A → ÷ M(A) → moles A → × (coef B ÷ coef A) → moles B → × M(B) → grams B

You never jump straight from grams of one substance to grams of another, because the balanced equation counts particles, not mass. One mole of propane reacts with five moles of oxygen, but 44 g of propane reacts with 160 g of oxygen: same ratio of molecules, very different ratio of masses.

Worked example

How much CO₂ forms when 44.1 g of propane burns completely? Moles of C₃H₈ = 44.1 ÷ 44.10 = 1.000 mol. The ratio CO₂ : C₃H₈ is 3 : 1, so 3.000 mol CO₂ form, which is 3.000 × 44.01 = 132 g.

Frequently asked questions

What is stoichiometry?
Stoichiometry is using the coefficients of a balanced equation as mole ratios to work out how much of one substance reacts with, or is produced from, a given amount of another.
What are the steps for a grams-to-grams stoichiometry problem?
1) Balance the equation. 2) Convert the known mass to moles with its molar mass. 3) Multiply by the mole ratio (coefficient of wanted ÷ coefficient of known). 4) Convert moles of the wanted substance back to grams with its molar mass.
Do I need to balance the equation first?
Yes, but this calculator does it for you. Type the unbalanced equation and the coefficients shown are the ones used in the ratios.
What if I have amounts for two reactants?
Then one of them probably runs out first. Use the limiting reactant calculator, which compares both and gives the theoretical yield.
How many significant figures should my answer have?
As many as the least precise measured value you started from. Molar masses and coefficients are treated as exact enough not to limit precision.