Limiting Reactant Calculator
Type a reaction and how much of each reactant you have, in grams, moles or litres of gas. The calculator balances the equation, finds the reactant that runs out first, and gives the theoretical yield of each product and the amount of every other reactant left over, with the working shown. 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
How much of each reactant do you have?
Leave it blank if N₂ is in unlimited supply, such as oxygen from the air.
Leave it blank if H₂ is in unlimited supply, such as oxygen from the air.
Used to turn litres of gas into moles and back, with V = RT/P. Only species written with (g) get a volume.
H₂
Limiting reactant
56.3 g
Theoretical yield of NH₃
Reactants
| Species | Available (mol) | Coefficient | Moles ÷ coefficient | Used (mol) | Left over (mol) | Left over (g) | Status |
|---|---|---|---|---|---|---|---|
| N₂ | 1.78 | 1 | 1.78 | 1.65 | 0.131 | 3.68 | In excess |
| H₂ | 4.96 | 3 | 1.65 | 4.96 | 0 | 0 | Limiting |
Theoretical yield
| Species | Moles (mol) | Mass (g) | Gas volume (L) |
|---|---|---|---|
| NH₃ | 3.31 | 56.3 | 75.1 |
Rounded to 3 significant figures, the fewest in the amounts you entered.
The working
n(N₂) = 50.00 g ÷ 28.01 g/mol = 1.785 mol
N₂: 1.785 mol ÷ 1 = 1.785
n(H₂) = 10.00 g ÷ 2.016 g/mol = 4.960 mol
H₂: 4.960 mol ÷ 3 = 1.653
The smallest quotient, 1.653, belongs to H₂, so it limits the reaction. Each product is its coefficient × 1.653.
How to Use the Limiting Reactant Calculator
- 1. Enter the reaction. Write it with an arrow, such as N2(g) + H2(g) -> NH3(g). It does not need to be balanced. Add (g) after a gas if you want its volume.
- 2. Enter how much of each reactant you have. Use a mass (mg, g, kg), an amount (mmol, mol) or a gas volume (mL, L), and choose the unit beside each box. Leave a box empty if that reactant is in unlimited supply, like oxygen from the air.
- 3. Read the limiting reactant. The headline shows which reactant runs out first and the mass of product it can make.
- 4. Check the tables and working. One table shows what is used and left over for each reactant; another gives the theoretical yield of each product in moles, grams and, for gases, litres.
How to Find the Limiting Reactant
The limiting reactant (or limiting reagent) is the one that is used up first, so it sets how much product can form. The reactant that is left over is in excess. You cannot tell which is limiting by comparing masses, or even moles, because the equation uses reactants in fixed proportions given by its coefficients.
The reliable method divides each reactant's moles by its coefficient. The result is how many times the reaction could run on that reactant alone, and the smallest value wins:
Moles
n = m ÷ M (or V ÷ molar volume for a gas)
Quotient
ξ(reactant) = n(reactant) ÷ coefficient
Limiting
ξ = the smallest quotient
Products
n(product) = coefficient × ξ
Excess left
n(reactant) − coefficient × ξ
Worked Example: 50.0 g of Nitrogen and 10.0 g of Hydrogen
The calculator opens on the Haber process, N₂ + 3H₂ → 2NH₃, with those masses.
- Nitrogen: 50.0 g ÷ 28.014 g/mol = 1.785 mol, and 1.785 ÷ 1 = 1.785
- Hydrogen: 10.0 g ÷ 2.016 g/mol = 4.960 mol, and 4.960 ÷ 3 = 1.653
- The smaller quotient is 1.653, so hydrogen is the limiting reactant
- Ammonia formed: 2 × 1.653 = 3.307 mol, a theoretical yield of 56.3 g
- Nitrogen used: 1.653 mol (46.3 g), so 1.785 − 1.653 = 0.131 mol, or 3.68 g, is left over
Nitrogen has five times the mass of the hydrogen, and hydrogen has the larger mole count (4.960 mol against 1.785 mol), yet hydrogen is the one that runs out, because the equation needs three moles of it for every mole of nitrogen. The theoretical yield you get here is the maximum that the Percent Yield Calculator compares your actual yield against.
Special Cases
- A tie. If two reactants have the same quotient, they are in exactly the proportion of the equation and run out together. The calculator marks both as limiting and reports nothing left over.
- An unlimited reactant. For a reaction in air, in a solvent or with a large excess of a reagent, leave that amount blank. It is left out of the comparison and shown as an unlimited supply.
- Solutions. If a reactant is given as a volume and concentration of solution, find its moles first with the Molarity Calculator and enter them in mol.
- Gases. A volume of gas needs the temperature and pressure, since the same moles occupy different volumes under different conditions.
Frequently Asked Questions
- How do you find the limiting reactant?
- Convert each reactant to moles, divide each by its coefficient in the balanced equation, and pick the smallest result. That reactant runs out first. An equivalent way is to calculate the product each reactant could make on its own; the one that makes the least is limiting.
- Is the limiting reactant the one with the smaller mass?
- Not necessarily. The reaction consumes moles in the ratio of the coefficients, and moles depend on molar mass. In the example above the nitrogen weighs five times more than the hydrogen, yet hydrogen is the limiting reactant because the equation needs three moles of it for each mole of nitrogen.
- How do you calculate how much excess reactant is left?
- Find the moles of the excess reactant that the limiting reactant consumes (its coefficient times the smallest quotient) and subtract that from the moles you started with. Multiply by the molar mass to get grams. The tables show both.
- What is the theoretical yield?
- It is the maximum amount of product the limiting reactant can make if the reaction goes to completion with no losses. It is the number you compare your measured yield against to get a percent yield.
- What if both reactants run out at the same time?
- Then the reactants were mixed in exactly the ratio of the coefficients. Neither is in excess, both are limiting, and the calculator shows nothing left over. This is also called a stoichiometric mixture.
- Can I use it when one reactant comes from the air?
- Yes. Leave the amount of that reactant blank and the calculator treats it as unlimited and leaves it out of the comparison. For a combustion, that gives the product from the fuel alone and shows how much oxygen the reaction would use.