Stoichiometry Calculator
Type a chemical equation, pick the species you know, and enter its mass, moles or volume of gas. The calculator balances the equation, then shows the moles, grams and litres of gas for every reactant and product, with the mole-ratio working written out line by line. Runs entirely in your browser. Free, no sign-up.
Balanced or not — the coefficients are worked out for you. Write ions with a sign (Fe3+, SO4^2-) and add (g) after a gas if you want its volume.
Results
Balanced equation
N2 + 3 H2 → 2 NH3
A mass, an amount in moles, or a volume of gas.
Used to turn litres of gas into moles and back, with V = RT/P. Only species written with (g) get a volume.
| Species | Role | Coefficient | Moles (mol) | Mass (g) | Gas volume (L) |
|---|---|---|---|---|---|
| N₂ | Reactant | 1 | 1.65 | 46.3 | 37.6 |
| H₂ | Reactant (given) | 3 | 4.96 | 10.0 | 113 |
| NH₃ | Product | 2 | 3.31 | 56.3 | 75.1 |
Rounded to 3 significant figures, the number in the amount you entered. The coefficients are exact and add none.
The working
n(H₂) = 10.00 g ÷ 2.016 g/mol = 4.960 mol
n(N₂) = 4.960 mol × (1 ÷ 3) = 1.653 mol
m(N₂) = 1.653 mol × 28.01 g/mol = 46.3 g
V(N₂) = 1.653 mol × 22.71 L/mol = 37.6 L
m(H₂) = 4.960 mol × 2.016 g/mol = 10.0 g
V(H₂) = 4.960 mol × 22.71 L/mol = 113 L
n(NH₃) = 4.960 mol × (2 ÷ 3) = 3.307 mol
m(NH₃) = 3.307 mol × 17.03 g/mol = 56.3 g
V(NH₃) = 3.307 mol × 22.71 L/mol = 75.1 L
How to Use the Stoichiometry Calculator
- 1. Enter the reaction. Write reactants, an arrow and products, such as N2(g) + H2(g) -> NH3(g). It does not need to be balanced, and any coefficients you type are ignored. Add (g) after a gas if you want its volume.
- 2. Say what you know. Choose the species and type its amount as a mass (mg, g, kg), an amount of substance (mmol, mol) or a volume of gas (mL, L).
- 3. Set the gas conditions if needed. For a volume, pick a standard condition or enter your own temperature and pressure; the molar volume comes from V = RT/P.
- 4. Read the table and the working. Every species gets its moles, mass and, for gases, volume, followed by the arithmetic so you can check each step.
The Mole-Ratio Method
Stoichiometry is bookkeeping for chemical reactions. The coefficients in a balanced equation say how many moles of each species react or form, so they are the exchange rate between substances. Every problem follows the same three steps: convert what you know to moles, apply the mole ratio, and convert the moles you want to the unit you need.
Mass to moles
n = m ÷ M
Mole ratio
n(B) = n(A) × coefficient of B ÷ coefficient of A
Moles to mass
m = n × M
Moles to gas volume
V = n × RT ÷ P
The ratio is only valid for a balanced equation, which is why the calculator balances yours first. If the equation has no single balance, it says so instead of guessing; the Chemical Equation Balancer explains what is wrong.
Worked Example: Making Ammonia
The calculator opens on the Haber process with 10.0 g of hydrogen. The balanced equation is N₂ + 3H₂ → 2NH₃, and hydrogen has a molar mass of 2.016 g/mol.
- Moles of hydrogen: 10.0 g ÷ 2.016 g/mol = 4.960 mol
- Moles of nitrogen needed: 4.960 mol × (1 ÷ 3) = 1.653 mol, which is 46.3 g at 28.014 g/mol
- Moles of ammonia formed: 4.960 mol × (2 ÷ 3) = 3.307 mol, which is 56.3 g at 17.031 g/mol
Mass is conserved: 10.0 g of hydrogen plus 46.3 g of nitrogen gives 56.3 g of ammonia. At IUPAC standard conditions (0 °C and 100 kPa, 22.711 L/mol) the three gases occupy 37.6 L of nitrogen, 113 L of hydrogen and 75.1 L of ammonia. Note that the answers carry three significant figures because 10.0 g does.
Gas Volumes and Standard Conditions
A mole of any ideal gas occupies V = RT/P, where R = 8.314 462 618… J/(mol·K) is exact since the 2019 SI redefinition. The value depends on the conditions, and textbooks do not agree on which are standard, so the calculator lets you choose:
| Conditions | Molar volume (L/mol) |
|---|---|
| 0 °C and 100 kPa — IUPAC standard (STP) | 22.711 |
| 0 °C and 1 atm (101.325 kPa) — older convention | 22.414 |
| 25 °C and 100 kPa — SATP | 24.790 |
| 25 °C and 1 atm | 24.465 |
IUPAC now recommends against using 1 atm as the standard pressure, but many courses still teach 22.4 L/mol. Use the condition your course specifies. Only species written with (g) get a volume, since the formula for a liquid or solid does not tell you its volume.
Common Mistakes
- Using an unbalanced equation. The mole ratio comes from the coefficients, so an unbalanced equation gives the wrong ratio.
- Using the ratio of masses. Coefficients compare moles, not grams. Convert to moles first.
- Inverting the ratio. Put the coefficient of the species you want on top and the species you know on the bottom.
- Applying molar volume to liquids and solids. The 22.4 L/mol figure is only for gases at the stated conditions.
- Rounding midway. Keep extra digits in the working and round once at the end.
Frequently Asked Questions
- How do you do stoichiometry step by step?
- Balance the equation, convert the known amount to moles, multiply by the mole ratio from the coefficients, then convert the moles you want to grams or litres. The calculator does all four steps and prints each line so you can compare it with your own working.
- How do you convert grams of A to grams of B?
- Divide the grams of A by its molar mass to get moles, multiply by (coefficient of B ÷ coefficient of A), then multiply by the molar mass of B. Pick A as the given species, type its mass, and the mass of B appears in the table.
- Why must the equation be balanced first?
- The coefficients are the mole ratios. In 2H₂ + O₂ → 2H₂O, two moles of hydrogen pair with one of oxygen; an unbalanced H₂ + O₂ → H₂O would suggest equal amounts and give wrong masses. The calculator balances it for you and shows the result.
- What is the molar volume of a gas at STP?
- It depends on which STP. At IUPAC's 0 °C and 100 kPa it is 22.711 L/mol; at the older 0 °C and 1 atm it is 22.414 L/mol, the 22.4 many textbooks print. Choose the conditions your course uses, or enter your own temperature and pressure.
- Can I use ions and electrons in the equation?
- Yes. Ions are accepted, written with a sign (Fe3+, SO4^2-), and a net ionic equation balances on charge as well as atoms. An ion's molar mass equals the neutral species' to the precision of atomic weights. Electrons (e-) show moles but no mass, because their mass is negligible.
- How many significant figures should the answer have?
- The same as the amount you entered, because the coefficients are exact and the atomic weights carry more digits. The calculator uses the digits in your amount, so 10.0 g gives three and 10 g gives two. Your instructor may ask you to follow the data with the fewest figures.
- How do I find the limiting reactant?
- When you know the amounts of two or more reactants, compare how much product each could make; the smaller amount is the limit. The Limiting Reactant Calculator does this directly, including how much of the excess reactant is left over.