Mole Ratio Calculator
The mole ratio is the ratio of coefficients between two species in a balanced equation. Multiply the known moles of one substance by (coefficient of target ÷ coefficient of given) to get moles of the other substance directly, no mass or molar mass needed. It is the middle step of a full theoretical yield calculation, used alone.
What a Mole Ratio Is
A mole ratio is the ratio of coefficients between any two species in a balanced chemical equation. It tells you how many moles of one substance react with, or produce, a given number of moles of another substance in the same reaction. Because the ratio comes straight from the coefficients, you can scale directly from moles of A to moles of B without touching mass, molar mass, or volume at any point.
This is different from full mass-to-mass stoichiometry, which starts with grams, converts to moles, applies the mole ratio, then converts back to grams. Here you already have moles of one substance and want moles of another, so the mole ratio step is the whole calculation. No molar mass is needed at all.
Reading Coefficients Off a Balanced Equation
Every species in a balanced equation has a coefficient, the number written directly in front of its formula. When no number appears in front of a formula, the coefficient is understood to be 1; it is simply never written out. In N2 + 3H2 -> 2NH3, nitrogen has an implicit coefficient of 1, hydrogen has a coefficient of 3, and ammonia has a coefficient of 2.
A mole ratio can be formed between any two species in the equation, not only between a named reactant and a named product. Reactant-to-reactant, product-to-product, and reactant-to-product ratios are all valid and all read the same way: put the coefficient of the substance you want (the target) over the coefficient of the substance you already have (the given). Every species, even one you were not directly asked about, still has an implicit ratio to every other species in the same equation.
How Mole Ratio Connects to Theoretical Yield
The mole ratio step is the middle step of the theoretical yield calculation. A full theoretical yield problem runs three steps in order: convert grams of the given substance to moles, multiply by the mole ratio to switch to moles of the target substance, then convert those moles back to grams using the target's molar mass. This calculator isolates that middle step so you can use it on its own whenever you already have moles of one substance, for example from a titration reading, a gas-law result, or one product of a multi-product reaction, and only need moles of a different species in the same equation.
Skipping straight to the mole ratio saves two conversion steps whenever mass was never part of the problem in the first place.
Worked Example 1: The Haber Process
Reaction: N2 + 3H2 -> 2NH3. Given: 0.500 mol N2. Find: mol NH3 produced.
- Coefficient of N2 is 1; coefficient of NH3 is 2. The mole ratio of NH3 to N2 is 2:1.
- n(NH3) = 0.500 mol N2 x (2 mol NH3 / 1 mol N2) = 1.00 mol NH3.
Every mole of nitrogen that reacts completely produces twice as many moles of ammonia, so the 2:1 ratio simply doubles the mole count.
Worked Example 2: Combustion of Propane
Reaction: C3H8 + 5O2 -> 3CO2 + 4H2O. Given: 0.200 mol C3H8. Find: mol CO2 produced.
- Coefficient of C3H8 is 1; coefficient of CO2 is 3. The mole ratio of CO2 to C3H8 is 3:1.
- n(CO2) = 0.200 mol C3H8 x (3 mol CO2 / 1 mol C3H8) = 0.600 mol CO2.
The same equation could also give moles of water or moles of oxygen consumed by swapping in the 4:1 or 5:1 ratios instead of the 3:1 ratio used here.
Worked Example 3: Decomposition of Potassium Chlorate
Reaction: 2KClO3 -> 2KCl + 3O2. Given: 0.150 mol KClO3. Find: mol O2 produced.
- Coefficient of KClO3 is 2; coefficient of O2 is 3. The mole ratio of O2 to KClO3 is 3:2.
- n(O2) = 0.150 mol KClO3 x (3 mol O2 / 2 mol KClO3) = 0.225 mol O2.
Neither coefficient here is 1, which shows the ratio still works the same way: divide by the given substance's coefficient, then multiply by the target's.
Quick Reference: Mole Ratios From These Equations
The table below collects several mole-ratio pairs pulled directly from the three equations above, useful for pattern recognition before plugging numbers into the calculator.
| Equation | Species A | Species B | Ratio A:B |
|---|---|---|---|
| N2 + 3H2 -> 2NH3 | N2 | NH3 | 1:2 |
| N2 + 3H2 -> 2NH3 | H2 | NH3 | 3:2 |
| C3H8 + 5O2 -> 3CO2 + 4H2O | C3H8 | CO2 | 1:3 |
| C3H8 + 5O2 -> 3CO2 + 4H2O | C3H8 | H2O | 1:4 |
| C3H8 + 5O2 -> 3CO2 + 4H2O | O2 | CO2 | 5:3 |
| 2KClO3 -> 2KCl + 3O2 | KClO3 | O2 | 2:3 |
| 2KClO3 -> 2KCl + 3O2 | KClO3 | KCl | 1:1 |
When to Use This Calculator Instead of Full Stoichiometry
Use this mole ratio calculator whenever the amount you already have is in moles and the amount you need is also in moles, with no mass conversion involved. Common cases include titration calculations where volume and concentration already give moles of one reagent, gas-law problems that output moles directly, and multi-product reactions where you know the moles of one product and want another.
If you are instead starting from grams of one substance and need grams of another, use the full stoichiometry calculator, which chains a molar mass conversion before and after this same mole-ratio step. If your end goal is the maximum mass a reaction can produce, the theoretical yield calculator runs all three steps together, and the general theory behind percent yield is covered on the percent yield calculator home page.
Frequently asked questions
What is a mole ratio?
A mole ratio is the ratio between the coefficients of two species in a balanced chemical equation. It shows how many moles of one substance correspond to a given number of moles of another substance in the same reaction, letting you scale directly between the two without converting to mass or volume at any point.
How do you find the mole ratio from a balanced equation?
Find it by taking the coefficient of the substance you want (the target) and dividing by the coefficient of the substance you already have (the given). In 2H2 + O2 -> 2H2O, the mole ratio of H2O to O2 is 2:1, read straight off the balanced coefficients with no calculation needed beyond identifying the two numbers.
Does the mole ratio use mass or moles?
The mole ratio uses moles only, never mass. It is built entirely from the coefficients in a balanced equation, which count particles in fixed proportions, not grams. Converting to or from mass requires a separate step using molar mass, which is not part of the mole ratio calculation itself.
What if a coefficient is not written in the equation?
A coefficient that is not written is understood to be 1; chemists omit the 1 rather than writing it out. In N2 + 3H2 -> 2NH3, nitrogen's coefficient is 1 even though no number appears before it. When forming a mole ratio involving that species, use 1 as its coefficient exactly as if it were printed.
Can you find the mole ratio between two products, not just reactant to product?
Yes, a mole ratio can be formed between any two species in the same balanced equation, including two products or two reactants. In C3H8 + 5O2 -> 3CO2 + 4H2O, the ratio between the two products CO2 and H2O is 3:4, found the same way as any reactant-to-product ratio, by comparing coefficients directly.
How does mole ratio connect to theoretical yield?
The mole ratio is the middle of the three steps used to calculate theoretical yield. A full theoretical yield problem converts grams to moles, applies the mole ratio to switch substances, then converts back to grams using molar mass. This calculator isolates that middle step for cases where you already have moles and only need the ratio conversion, without any mass involved.
What happens if you use the wrong mole ratio?
Using the wrong mole ratio gives an incorrect mole amount for the target substance, which then carries through every later step of the calculation, including any mass or yield figure derived from it. A common mistake is inverting the ratio (given over target instead of target over given) or misreading a coefficient, both of which scale the answer by the wrong factor.
Can a mole ratio be a fraction like 3 to 2?
Yes, a mole ratio does not need to simplify to a whole number; 3:2, 5:3, and similar fractional ratios are common and perfectly valid. The decomposition of potassium chlorate, 2KClO3 -> 2KCl + 3O2, gives a KClO3 to O2 ratio of 2:3, and the calculation multiplies through by that fraction exactly as written.
Do you need to balance the equation before using mole ratios?
Yes, the equation must be balanced first, because mole ratios are only valid when coefficients reflect a properly balanced reaction. An unbalanced equation gives coefficients that do not represent the true proportion of reacting particles, so any ratio taken from it will be wrong. Balance the equation before using this calculator.
How is mole ratio different from mass ratio?
A mole ratio compares the number of moles (particles) of two substances, while a mass ratio compares their masses in grams. Because different substances have different molar masses, equal mole amounts of two substances almost never have equal mass, so the two ratios are generally different numbers for the same pair of substances in the same reaction.