Stoichiometry of the Reaction

General Chemistry • Chemical Reactions

Written by STEM Calculators Team Published October 17, 2025 Updated February 24, 2026

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Stoichiometric Ratios — Using a Balanced Equation

Once a chemical equation is balanced, its coefficients are stoichiometric numbers (mole ratios). They let you convert moles of one substance to moles of any other substance in the reaction.

General relationships

\[ a\,\mathrm{A} + b\,\mathrm{B} \;\longrightarrow\; c\,\mathrm{C} + d\,\mathrm{D} \]

For any balanced reaction, the following ratios are equal:

\[ \frac{n_{\mathrm{A}}}{a} \;=\; \frac{n_{\mathrm{B}}}{b} \;=\; \frac{n_{\mathrm{C}}}{c} \;=\; \frac{n_{\mathrm{D}}}{d} \]

If you choose a reference species “ref” with coefficient \(\nu_{\text{ref}}\), then for any species \(X\):

\[ n_X \;=\; n_{\text{ref}} \times \frac{\nu_X}{\nu_{\text{ref}}} \]

How to use the calculator

Enter the unbalanced equation using + and -> (or →).
Press Balance & build ratios to get the balanced equation and coefficients.
Pick a reference species and type its amount in moles.
The tool computes moles of every other species using the coefficient ratios.

Worked example 1

Iron(III) oxide formation (balanced): 4 Fe + 3 O₂ → 2 Fe₂O₃

Given (reference):

\[ n_{\text{ref}} = n_{\mathrm{O_2}} = 2.50~\text{mol},\qquad \nu_{\text{ref}} = \nu_{\mathrm{O_2}} = 3 \]

Moles of Fe needed:

\[ n_{\mathrm{Fe}} = n_{\text{ref}} \times \frac{\nu_{\mathrm{Fe}}}{\nu_{\text{ref}}} = 2.50 \times \frac{4}{3} = 3.33~\text{mol} \]

Moles of Fe\(_2\)O\(_3\) produced:

\[ n_{\mathrm{Fe_2O_3}} = 2.50 \times \frac{2}{3} = 1.67~\text{mol} \]

Common pitfalls

Do not change subscripts to balance; change only the coefficients.
Be sure the equation is actually balanced before using ratios.
State symbols (s, l, g, aq) don’t affect the stoichiometric math.

Frequently Asked Questions

How does the Stoichiometry of the Reaction calculator convert moles between species?

It uses the balanced equation coefficients as mole ratios. If a reference species is known, each other species is found with n_X = n_ref x (nu_X / nu_ref).

Do I need to balance the equation before using mole ratios?

Yes. Mole ratios are valid only for a balanced equation because balancing ensures the coefficients correctly represent conservation of atoms for the reaction.

Can I enter grams instead of moles in this calculator?

This calculator is designed for mole-to-mole stoichiometry using coefficients. If you need mass-to-mass or mass-to-mole conversions, use a tool that includes molar mass conversions.

Why are charges, electrons, and half-reactions not supported?

This calculator balances standard chemical equations by atom conservation and uses coefficient mole ratios. Redox half-reaction balancing requires tracking charge and electrons, which is outside this tool’s scope.

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