Stoichiometry Calculator
Stoichiometry uses the ratios in a balanced equation to work out how much of each substance a reaction consumes and produces.
Formula
moles of product = moles of reactant × (product coefficient ÷ reactant coefficient)
- Coefficients apply to moles, never to grams
- The reactant that runs out first is the limiting reagent
N₂ + 3H₂ → 2NH₃ with 1.5 mol of H₂: 1.5 × 2/3 = 1 mol of NH₃
The Method
Every calculation follows the same three steps:
- Convert to moles: mass ÷ molar mass
- Apply the mole ratio from the balanced equation
- Convert back to whatever unit you need
In 2H₂ + O₂ → 2H₂O, two moles of hydrogen react with one of oxygen. Four moles of hydrogen therefore need two of oxygen and yield four of water.
The equation must be balanced first, or every ratio that follows is wrong.
The Limiting Reagent
Reactions stop when one reactant runs out, regardless of how much of the others remain. That reactant is limiting, and it alone determines the yield.
To find it, convert each reactant to moles and divide by its coefficient in the equation. The smallest result is limiting. Comparing raw masses is unreliable, because molar masses differ.
Everything left over is in excess, which is often deliberate: chemists frequently use surplus of the cheaper reactant to drive the expensive one to completion.
Theoretical Versus Actual Yield
Percentage yield = (Actual ÷ Theoretical) × 100
Real reactions never reach the theoretical figure. Losses come from incomplete reaction, competing side reactions, and material left behind during transfer and purification.
A yield above 100% does not mean success — it means the product is still wet with solvent or contaminated, and needs drying and reweighing.
Quick Reference
| Step | Operation | Worked |
|---|---|---|
| 1. Grams to moles | mass ÷ molar mass | 3.02 g H₂ ÷ 2.016 = 1.5 mol |
| 2. Apply the ratio | × product ÷ reactant | 1.5 × 2/3 = 1 mol NH₃ |
| 3. Moles to grams | × molar mass | 1 × 17.03 = 17.03 g |
| 4. Percentage yield | actual ÷ theoretical | 14.2 ÷ 17.03 = 83.4% |
Frequently Asked Questions
- What is a limiting reagent?
- The reactant that runs out first and therefore caps the yield. Find it by dividing each reactant's moles by its coefficient and taking the smallest result.
- How do I calculate percentage yield?
- Divide actual yield by theoretical yield and multiply by 100. Real reactions rarely exceed 90 per cent.
- Why must the equation be balanced first?
- The coefficients are the mole ratios. An unbalanced equation gives the wrong ratios, so every quantity derived from it is wrong.
- Can percentage yield exceed 100 per cent?
- Not genuinely. A figure above 100 means the product retains solvent or impurities and needs drying before reweighing.