Limiting Reagent Calculator
Which of two reagents runs out first, how much of the other is left over, and how far the reaction goes.
Results
What this tool does
Take the coefficients straight from the balanced equation. The result marked 1 or 2 tells you which reagent limits the reaction; the excess mass is what will still be in the flask when it stops. Everything after that — the yield, the product mass — is set by the limiting one alone.
Formula
compares n ÷ coefficient for each reactant ; the smallest one limits
Variables
| Symbol | Meaning | Unit |
|---|---|---|
m1 | Mass of first reagent | g |
w1 | Molar mass | g/mol |
k1 | Stoichiometric coefficient | — |
m2 | Mass of second reagent | g |
w2 | Molar mass | g/mol |
k2 | Stoichiometric coefficient | — |
R1 | Moles per coefficient | — |
R2 | Moles per coefficient | — |
LM | Limiting reagent (1 or 2) | — |
EX | Excess reagent left | g |
EN | Amount of substance | mol |
Worked example
- Mass of first reagent10 g
- Molar mass2.016 g/mol
- Stoichiometric coefficient2
- Mass of second reagent64 g
- Molar mass31.998 g/mol
- Stoichiometric coefficient1
- Moles per coefficient2.480159
- Moles per coefficient2.000125
- Limiting reagent (1 or 2)2
- Excess reagent left1.93550 g
- Amount of substance2.000125 mol
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
Frequently asked questions
Why divide the moles by the coefficient?
Because having more moles does not mean having more to spare. In 2H₂ + O₂ → 2H₂O, every oxygen molecule needs two hydrogens, so a mole of oxygen goes twice as far as a mole of hydrogen. Dividing by the coefficient puts both on the same footing: whichever result is smaller runs out first, no matter which had more moles to begin with.