The molar (mole) ratio is the bridge in every stoichiometry problem. The coefficients of a balanced equation say how many moles of each substance react or form together: in N₂ + 3 H₂ → 2 NH₃, one mole of nitrogen reacts with three moles of hydrogen to give two moles of ammonia, so the N₂ : H₂ : NH₃ ratio is 1 : 3 : 2. Any amount of one species can be converted into the amount of another by multiplying by the ratio of their coefficients.
The ratio is a ratio of moles, not of masses. Because each substance has a different molar mass, the mass ratio is quite different – in the ammonia example 1 g of hydrogen combines with 4.63 g of nitrogen. Mixing up the two is one of the most common stoichiometry mistakes. For gases at the same temperature and pressure, though, the mole ratio is also the volume ratio (Avogadro’s law).
Type an equation – balanced or not – and pick a reference species. The calculator balances it, lists the mole ratios and mass ratios, and in the other modes turns a known number of moles or grams of the reference species into the moles and masses of everything else.
How to use the Molar Ratio Calculator
- Chemical equation (balanced or not) — Use -> or = between the two sides
- I want to calculate
- Known (reference) species — Type it as it appears in the equation
- Amount of the reference species
- Mass of the reference species
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
The equation is balanced with exact integer arithmetic. Each species’ coefficient divided by the reference species’ coefficient gives moles per mole of the reference; multiplying by molar masses (IUPAC standard atomic weights) gives grams per gram. With a known amount, the moles of every species follow from those ratios.
Worked example
Ammonia synthesis: N2 + H2 → NH3 balances to N₂ + 3 H₂ → 2 NH₃, so the molar ratio is 1 : 3 : 2. Starting from 6.0 mol of H₂, you need 6.0 × 1/3 = 2.0 mol N₂ (56.03 g) and form 6.0 × 2/3 = 4.0 mol NH₃ (68.12 g).
With 12.0 g of H₂ instead (12.0 ÷ 2.016 = 5.952 mol), the same ratios give 1.984 mol (55.58 g) N₂ and 3.968 mol (67.58 g) NH₃ – and the masses balance: 55.58 + 12.00 = 67.58 g.
Mole ratios in common reactions
| Reaction | Balanced equation | Mole ratio |
|---|---|---|
| Water formation | 2 H₂ + O₂ → 2 H₂O | 2 : 1 : 2 |
| Methane combustion | CH₄ + 2 O₂ → CO₂ + 2 H₂O | 1 : 2 : 1 : 2 |
| Photosynthesis | 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ | 6 : 6 : 1 : 6 |
| Table salt | 2 Na + Cl₂ → 2 NaCl | 2 : 1 : 2 |
Formula
mol B = mol A × (coefficient of B ÷ coefficient of A) · mass B = mol B × M(B)
Worked example
Inputs — Chemical equation (balanced or not): N2 + H2 -> NH3; I want to calculate: Molar ratio and moles; Known (reference) species: H2; Amount of the reference species: 6 mol.
Results — Molar ratio = N₂ : H₂ : NH₃ = 1 : 3 : 2; Balanced equation = N₂ + 3 H₂ → 2 NH₃; Per mole of the reference species = For every 1 mol of H₂: 0.3333 mol N₂, 0.6667 mol NH₃; Mass ratio (relative to the reference species) = N₂ : H₂ : NH₃ = 4.6319 : 1 : 5.6319; Moles of the reference species = 6; Moles and masses of every species = Species Coefficient mol per mol H₂ M (g/mol) g per g H₂ Moles (mol) Mass (g)
──────────────────────────────────────────────────────────────────────────────────────────────
N₂ (reactant) 1 0.3333 28.014 4.6319 2 56.028
H₂ (reactant) 3 1 2.016 1 6 12.096
NH₃ (product) 2 0.6667 17.031 5.6319 4 68.124.
Key terms
- Mole ratio
- The ratio of the coefficients of two species in a balanced equation.
- Stoichiometric coefficient
- The number in front of a formula in a balanced equation.
- Molar mass
- The mass of one mole of a substance, in g/mol.
- Avogadro's law
- Equal volumes of ideal gases at the same T and p contain equal numbers of molecules.
Frequently asked questions
What is a molar ratio?
The ratio of the amounts (in moles) of any two substances in a balanced equation, read straight from their coefficients. In 2 H₂ + O₂ → 2 H₂O the H₂ : O₂ ratio is 2 : 1.
How do I find the mole ratio?
Balance the equation first, then compare coefficients. The calculator does both: N2 + H2 -> NH3 becomes N₂ + 3 H₂ → 2 NH₃, giving 1 : 3 : 2.
How do I use a mole ratio to convert amounts?
Multiply the known moles by (coefficient wanted ÷ coefficient known). From 6 mol H₂: N₂ = 6 × 1/3 = 2 mol, NH₃ = 6 × 2/3 = 4 mol.
Is the mole ratio the same as the mass ratio?
No. In the ammonia synthesis the mole ratio N₂ : H₂ is 1 : 3, but the mass ratio is 28.01 : 6.05 ≈ 4.63 : 1.
Is the mole ratio the same as the volume ratio for gases?
Yes, for ideal gases at the same temperature and pressure, by Avogadro's law: 1 L of N₂ reacts with 3 L of H₂.
How does the mole ratio identify the limiting reagent?
Divide each reactant's moles by its coefficient; the smallest result is the limiting reagent. Use the theoretical yield calculator to do this automatically.