Relating the Mass of Reactants and Products

General Chemistry • Chemical Reactions

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

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Relating the Mass of Reactants and Products

Once a chemical equation is balanced, its coefficients give mole ratios. To relate masses, convert grams → moles using the molar mass (g·mol⁻¹), use the mole ratio, then convert moles → grams.

Workflow

Balance the equation (do not change subscripts).
Compute molar masses of each species from atomic masses (sum of atoms × atomic mass).
Pick a reference species and an amount in grams.
Convert to moles: n(ref) = m(ref) / M(ref).
Use mole ratios: for any species B, n(B) = n(ref) × coef(B)/coef(ref).
Convert to mass: m(B) = n(B) × M(B).

Key formulas (LaTeX)

\[ n_{\text{ref}}=\frac{m_{\text{ref}}}{M_{\text{ref}}},\qquad n_{B}=n_{\text{ref}}\cdot\frac{\nu_{B}}{\nu_{\text{ref}}},\qquad m_{B}=n_{B}\,M_{B} \]

where \( \nu \) are stoichiometric coefficients from the balanced equation, \( M \) are molar masses (g·mol\(^{-1}\)), \( n \) are moles, and \( m \) are masses (g).

Example

Combustion of propane: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

Take 44.1 g C₃H₈ (M ≈ 44.1 g·mol⁻¹).

\[ \begin{aligned} n(\text{C}_3\text{H}_8) &= \frac{44.1\ \text{g}}{44.1\ \text{g·mol}^{-1}} = 1.00\ \text{mol} \\ n(\text{CO}_2) &= 1.00 \times \frac{3}{1} = 3.00\ \text{mol} \\ m(\text{CO}_2) &\approx 3.00 \times 44.01 = 132\ \text{g} \\ n(\text{H}_2\text{O}) &= 1.00 \times \frac{4}{1} = 4.00\ \text{mol} \\ m(\text{H}_2\text{O}) &\approx 4.00 \times 18.02 = 72.1\ \text{g} \end{aligned} \]

Tips & Guardrails

Use + between species and -> (or →) between sides.
Parentheses are handled: Fe₂(SO₄)₃, Ba(NO₃)₂, Ca(OH)₂.
Optional state symbols are ignored; charges/electrons and half-reactions are not supported.
Mass is conserved: the computed total mass of reactants equals that of products for the chosen scale.

Out of scope

Limiting reagent analysis (needs initial amounts of multiple reactants).
Percent yield (needs actual yield) and molar-mass-from-mass-percent problems.

Frequently Asked Questions

How does this calculator relate the mass of reactants and products?

It balances the equation first, then converts a known mass to moles with n = m / M, scales all species using coefficient mole ratios, and converts back to grams with m = n x M.

Why must the chemical equation be balanced before doing mass calculations?

Only a balanced equation has coefficients that represent correct mole ratios between species. Using an unbalanced equation would give incorrect stoichiometric relationships and wrong masses.

What is the key formula used to convert grams to moles in stoichiometry?

The conversion is n = m / M, where n is moles, m is mass in grams, and M is molar mass in g/mol. The calculator then uses coefficient ratios to find moles of other species.

Can this calculator do limiting reactant or percent yield problems?

No. Limiting reactant requires starting amounts for multiple reactants, and percent yield requires an actual yield; this tool focuses on mass relationships from one given reference mass.

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Frequently Asked Questions

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