Limiting Reactant Calculator
Limiting reactant, excess left, and theoretical yield from masses.
What it does
Limiting reactants decide how far a reaction goes: divide each reactant moles by its equation coefficient and the smaller quotient runs out first. Everything else follows from that bottleneck: excess leftovers and theoretical product.
The tool above takes both masses with molar masses and the three coefficients. It returns which reactant limits with excess moles left and theoretical product moles beside them.
The default 10 g H2 with 80 g O2 in 2H2 + O2 to 2H2O gives 4.96/2 = 2.48 against 2.5/1 = 2.50: A limits, leaving 0.02 mol O2 with 4.96 mol water made.
Stoichiometry follows OpenStax Chemistry chapters with the same coefficient division in Khan Academy units. Both assume complete reaction with balanced equations entered correctly.
How to use
- Enter both reactant masses in grams.
- Enter both molar masses.
- Enter the balanced equation coefficients.
- Enter the product coefficient for yield.
- Read limiting first, then excess and product.
Formula
- moles/coef
- Reaction portions each reactant can run.
- smaller
- Limiting reactant running out first.
- theoretical
- Product at 100% conversion.
Coefficients must come from a balanced equation; guessing ratios corrupts every output. Percent yield divides actual product by this theoretical value on the bench.
Worked example
Example: 10 g H2 + 80 g O2 (2H2 + O2)
Defaults: 10 g at 2.016, 80 g at 32.00, coefficients 2, 1, 2.
- Moles: 10 / 2.016 = 4.96 mol H2. 80 / 32 = 2.5 mol O2.
- Portions: 4.96 / 2 = 2.48 vs 2.5 / 1 = 2.50.
- Smaller is A: hydrogen limits.
- O2 needed: 4.96 / 2 = 2.48. Left: 2.5 - 2.48 = 0.02 mol.
- Water: 4.96 / 2 x 2 = 4.96 mol. Tool shows A, 0.02, 4.96.
Reference table
| Mix | Limits | Product |
|---|---|---|
| 10 g H2 + 80 g O2 | A (2.48 vs 2.50) | 4.96 mol |
| 4 g H2 + 32 g O2 | A (0.99 vs 1.00) | 1.98 mol |
| 2 g H2 + 10 g O2 | B (0.5 vs 0.31) | 0.625 mol |
Limits
- Complete reaction assumed; real yields run lower.
- Balanced equations required from the user.
- Side reactions ignored.
- Purity scales grams first when technical grade.
Common mistakes
- Comparing grams instead of moles per coefficient.
- Forgetting to divide by coefficients.
- Using unbalanced guess ratios.
- Reading excess as product.
- Expecting theoretical yields on the bench.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 g H2 + 32 g O2?
1.984/2 = 0.992 vs 1: A limits, 0.008 O2 left, 1.984 water.
Excess when exactly balanced?
Zero left: portions tie, both run out together.
Product per A portion?
Portions times product coef: 2.48 x 2 = 4.96 here.
Questions
How do you find limiting reactant?
Moles over coefficients; the smaller quotient limits.
What is theoretical yield?
Product at full conversion of the limiting reactant.
Why compare per coefficient?
Recipes consume fixed ratios; raw moles mislead.
Excess vs limiting?
Limiting runs out; excess remains after.
Do I need balanced equations?
Yes. Coefficients come only from balanced forms.
Why lower real yields?
Side paths, equilibrium, and bench losses take their share.