In most reactions the reactants are not mixed in the exact proportions the equation calls for. One of them runs out first, and the moment it is gone the reaction stops, no matter how much of the others remains. That reactant is the limiting reactant, and it sets the theoretical yield. This calculator reads a chemical equation as you type it, balances it if necessary, converts each reactant to moles, identifies the limiting one and reports the yield of every product and the amount of each excess reactant left over.
How to use the limiting reactant calculator
- Type the Chemical equation, for example
CH4 + 2O2 -> CO2 + 2H2O. Use->,=or→between the sides and+between species. Formulas may include parentheses and hydrates. - Leave Balance the equation for me ticked if you have not written the coefficients, or untick it to have your coefficients checked instead.
- Enter the amount of each reactant in the order written: 1st reactant, 2nd reactant and, for three-reactant equations, the 3rd reactant. Each accepts g, mg, kg, lb, oz, mol or mmol.
- Read the limiting reactant, the theoretical yield of each product in grams and moles, the leftover excess reactants and a mass-balance check. Two tables summarize every species.
How the calculation works
For each reactant i with amount ni in moles and coefficient νi:
ξ is the extent of reaction, the number of times the reaction “as written” can run. The reactant that gives the minimum is limiting. Each product j then forms
and each excess reactant has ni − νiξ moles left. Masses are converted with n = m ÷ M, using IUPAC standard atomic weights for the molar masses. Mass is conserved, so the grams of reactants consumed always equal the grams of products formed.
Worked example
Burning methane
CH4 + 2O2 → CO2 + 2H2O, starting with 16 g of methane and 48 g of oxygen.
Moles: CH4 = 16 ÷ 16.043 = 0.9973 mol; O2 = 48 ÷ 31.998 = 1.5001 mol.
Divide by coefficients: CH4 0.9973 ÷ 1 = 0.9973; O2 1.5001 ÷ 2 = 0.7500. Oxygen is limiting.
Products: CO2 = 0.7500 × 44.009 = 33.01 g; H2O = 1.5001 × 18.015 = 27.02 g. Unreacted methane: 0.2473 mol, or 3.97 g.
Notice that there is three times as much oxygen as methane by mass, yet oxygen still runs out, because each methane molecule needs two oxygen molecules and oxygen molecules are twice as heavy.
Making ammonia. Enter N2 + H2 -> NH3 with 50 g of nitrogen and 10 g of hydrogen. The calculator balances it to N2 + 3H2 → 2NH3, finds that hydrogen is limiting (4.96 mol ÷ 3 = 1.65 versus 1.78 for nitrogen) and leaves 3.68 g of nitrogen unreacted.
Tips for entering equations
| You type | The calculator reads |
|---|---|
2H2 + O2 = 2H2O |
Coefficients 2, 1, 2 |
Ca(OH)2 + HCl -> CaCl2 + H2O |
Balanced to 1, 2, 1, 2 |
CuSO4·5H2O or CuSO4*5H2O |
A hydrate with five waters |
Ag+ + Cl- -> AgCl |
Ion charges are ignored for masses |
Element symbols are case-sensitive: Co is cobalt, while CO is carbon monoxide. Each species may appear only once, and every element must appear on both sides, otherwise no coefficients can balance the equation.
From theoretical to actual yield
The theoretical yield is an upper limit. Real yields are lower, and chemists judge a procedure by its percent yield; carry the theoretical value to the percent yield calculator to compare it with what you actually collected. For a single compound’s molar mass, use the molar mass calculator, and for gram–mole conversions on their own, the grams to moles calculator.
Frequently asked questions
How do you find the limiting reactant?
Convert each reactant to moles, then divide by its coefficient in the balanced equation. The reactant with the smallest result runs out first and limits how much product can form. Comparing grams directly gives the wrong answer because molar masses and coefficients differ.
Does the equation have to be balanced?
The math needs a balanced equation, but you do not have to balance it yourself. With “Balance the equation for me” ticked, the calculator finds the smallest whole-number coefficients, for example turning Al + Cl2 = AlCl3 into 2Al + 3Cl2 → 2AlCl3.
Can I enter moles instead of grams?
Yes. Each amount has its own unit menu with g, mg, kg, lb, oz, mol and mmol, and you can mix them. The calculator converts masses to moles with standard atomic weights.
What if both reactants run out at the same time?
Then the mixture is stoichiometric and both are limiting. For 4Fe + 3O2 → 2Fe2O3, 1 mol of iron and 0.75 mol of oxygen react completely to give 0.5 mol (79.84 g) of iron(III) oxide with nothing left over.
Why is my real yield lower than the theoretical yield?
The theoretical yield assumes every molecule of the limiting reactant reacts exactly as written. In practice reactions stop short of completion, side reactions use up material and some product is lost while it is collected. The percent yield calculator compares the two.