Stoichiometry Calculator
Convert between moles and mass in chemical reactions using stoichiometric ratios, and identify the limiting reagent.
What Is a Stoichiometry Calculator?
Stoichiometry is the quantitative study of chemical reactions — specifically the relationships between the amounts of reactants consumed and products formed. The word comes from the Greek stoicheion (element) and metron (measure). Every balanced chemical equation is a stoichiometric statement: 2H₂ + O₂ → 2H₂O tells us that exactly two moles of hydrogen react with one mole of oxygen to produce two moles of water.
This calculator handles the three most common stoichiometry problems students and chemists encounter:
- Mole Ratio: Convert moles of one substance to moles of another using the equation's coefficients.
- Mass Calculation: Convert grams of a reactant to grams of a product (the full mole roadmap: g → mol → mol → g).
- Limiting Reagent: Determine which reactant runs out first and how much of the other remains in excess.
Understanding stoichiometry is essential for laboratory work, industrial process chemistry, pharmaceutical manufacturing, and environmental analysis — any field where chemical quantities must be precisely controlled or predicted.
Stoichiometry Formulas
All stoichiometry is built on three relationships:
| Calculation | Formula | When Used |
|---|---|---|
| Moles from mass | n = m / MM | Converting grams to moles (MM = molar mass in g/mol) |
| Mole ratio | n_B = n_A × (coef_B / coef_A) | Converting moles of A to moles of B |
| Mass from moles | m = n × MM | Converting moles back to grams |
| Limiting reagent | Compare n_A / coef_A vs n_B / coef_B | Smallest ratio → limiting reagent |
| Theoretical yield | m_product = n_limiting × (coef_P / coef_L) × MM_P | Max product from limiting reagent |
| Percent yield | % = (actual / theoretical) × 100 | Measuring reaction efficiency |
The universal stoichiometry roadmap: grams A → (÷ MM_A) → moles A → (× ratio) → moles B → (× MM_B) → grams B
📖 How to Use the Stoichiometry Calculator
Steps to Calculate
Tip: Use the Molecular Weight Calculator to find molar masses from chemical formulas before doing a mass calculation.
Example Calculations
Example 1 — Mole Ratio (Combustion of Methane)
CH₄ + 2O₂ → CO₂ + 2H₂O — How many moles of O₂ react with 3 mol CH₄?
Example 2 — Mass Calculation (Synthesis of Water)
2H₂ + O₂ → 2H₂O — How many grams of water form from 10 g of H₂?
Example 3 — Limiting Reagent (Haber Process)
N₂ + 3H₂ → 2NH₃ — 4 mol N₂ and 9 mol H₂ available. Which is limiting?
❓ Frequently Asked Questions
🔗 Related Calculators
What is stoichiometry?
Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction, based on the law of conservation of mass. It uses balanced chemical equations and molar masses to calculate how much of each substance reacts or is produced. The stoichiometric coefficients in a balanced equation give the mole ratios: for 2H₂ + O₂ → 2H₂O, two moles of H₂ react with one mole of O₂ to produce two moles of H₂O.
What is a mole ratio?
A mole ratio is the ratio of the stoichiometric coefficients from a balanced equation, used to convert between moles of different substances. For the reaction aA + bB → cC, the mole ratio of A to B is a:b. If you have n_A moles of A, the moles of B consumed = n_A × (b/a), and moles of C produced = n_A × (c/a). Mole ratios are exact — they come from the integer coefficients in the balanced equation.
What is a limiting reagent?
The limiting reagent (or limiting reactant) is the reactant that is completely consumed first in a reaction, thereby limiting how much product can be formed. The other reactant is the excess reagent — some of it is left over after the reaction. To find the limiting reagent: divide the available moles of each reactant by its stoichiometric coefficient; the reactant with the smallest result is the limiting reagent.
How do you calculate theoretical yield?
Theoretical yield is the maximum amount of product that could form if the limiting reagent is fully consumed with no losses. Steps: (1) Identify the limiting reagent. (2) Use the mole ratio to find moles of product: n_product = n_limiting × (coef_product / coef_limiting). (3) Convert to grams: mass = moles × molar mass. The actual experimental yield is always ≤ theoretical yield.
What is the mole-to-mole conversion method?
Mole-to-mole conversion uses the balanced equation's coefficients as the conversion factor. For N₂ + 3H₂ → 2NH₃: if you start with 4 mol H₂, moles of NH₃ = 4 × (2/3) = 2.67 mol. The steps are always: (1) Start with known moles of substance A. (2) Multiply by (coef_B / coef_A). (3) The result is moles of substance B. No molar masses are needed for mole-to-mole calculations.
How do you do a mass-to-mass stoichiometry calculation?
Mass-to-mass conversion: (1) Convert mass of A to moles: n_A = mass_A / MM_A. (2) Apply mole ratio: n_B = n_A × (coef_B / coef_A). (3) Convert moles of B to mass: mass_B = n_B × MM_B. Example: how many grams of H₂O form from 36 g of H₂? MM(H₂) = 2 g/mol, MM(H₂O) = 18 g/mol. Moles H₂ = 36/2 = 18 mol. Moles H₂O = 18 × (2/2) = 18 mol. Mass H₂O = 18 × 18 = 324 g.
What is percent yield in chemistry?
Percent yield = (actual yield / theoretical yield) × 100%. It measures the efficiency of a reaction. Less than 100% is expected due to: incomplete reactions (equilibrium not reached), side reactions producing other products, physical losses during isolation/purification, and measurement errors. A 70–90% yield is considered good for complex organic syntheses; simple inorganic reactions can achieve >95%.
What is molar mass and how is it used in stoichiometry?
Molar mass is the mass (in grams) of one mole of a substance — numerically equal to the molecular weight in atomic mass units (amu). It serves as the conversion factor between grams and moles: moles = mass / molar mass. Examples: H₂O = 18.015 g/mol; NaCl = 58.44 g/mol; CO₂ = 44.01 g/mol. In stoichiometry, you convert grams → moles → (via mole ratio) → moles of product → grams of product.
How do you find the excess reagent amount?
After identifying the limiting reagent, calculate how much of the excess reagent was consumed: moles consumed = moles of limiting reagent × (coef_excess / coef_limiting). Subtract from the initial moles: moles remaining = initial moles − moles consumed. Multiply by molar mass for grams remaining. This tells you how much excess reagent is left over after the reaction is complete.
What is the difference between reactants and products in stoichiometry?
In a chemical equation A + B → C + D, A and B are reactants (starting materials consumed) and C and D are products (substances formed). Stoichiometry applies equally to all: mole ratios connect any pair of substances. The arrow direction does not affect the ratios — you can calculate how many moles of any reactant react with any amount of another reactant or product using the same mole ratio approach.
What does 'balanced equation' mean and why is it essential for stoichiometry?
A balanced equation has the same number of each type of atom on both sides of the reaction arrow, satisfying the law of conservation of mass. Example: unbalanced H₂ + O₂ → H₂O; balanced 2H₂ + O₂ → 2H₂O. The integer coefficients in the balanced equation give the exact mole ratios for stoichiometry calculations. Using unbalanced coefficients gives incorrect mole ratios and wrong answers. Always balance the equation before doing any stoichiometry.