Mass stoichiometry from a balanced equation
A balanced equation supplies mole ratios through its coefficients. Converting a known mass to moles allows those ratios to determine other reactant and product amounts.
The calculator verifies supported formulas and atom balance before calculating. It assumes complete conversion and does not choose a limiting reactant.
How to use the stoichiometry calculator
- Enter a balanced equation: Use -> and supported neutral formulas.
- Select a known species: Choose a reactant or product.
- Enter mass: Supply its known mass in grams.
- Calculate: Review corresponding moles and masses.
Formula and variables
Convert known mass to moles, apply the coefficient ratio, then convert target moles to mass.
nB = (mA/MA)(νB/νA); mB = nBMB- m — Mass
- Substance mass (g)
- M — Molar mass
- Mass per mole (g/mol)
- ν — Coefficient
- Balanced-equation coefficient
Water formation
Use 10 g H₂ in 2H₂ + O₂ → 2H₂O.
- Known mass
- 10 g H₂
- Convert H₂ mass to moles
- Apply 1:2 O₂:H₂ and 2:2 H₂O:H₂ ratios
Result: Approximately 79.36 g O₂ corresponds to 89.36 g H₂O.
This assumes enough oxygen and complete conversion.
Understanding your results
Theoretical relationship
Results are exact model amounts based on coefficients and representative molar masses.
- Use Limiting Reactant when several reactant masses are supplied.
- Use Percent Yield to compare theoretical and actual product.
Assumptions
- Equation is balanced and reaction proceeds as written.
- The known substance is pure.
- Representative average atomic weights are suitable.
Limitations
- Does not balance equations.
- Does not support charges, hydrates, isotope labels, or non-stoichiometric compounds.
- Does not model limiting reagents, equilibrium, or percent yield.
Common mistakes
- Using an unbalanced equation.
- Changing formula subscripts instead of coefficients.
- Applying mass ratios directly instead of mole ratios.
Practical use cases
Grams-to-grams problems
Calculate theoretical reactant requirements and product masses.
Frequently asked questions
Does this balance the equation?
No. It checks balance and reports an error when element totals differ.
Can the known substance be a product?
Yes. The coefficient relationship can be rearranged from any listed species.
Sources and review
- Reaction Stoichiometry — OpenStax Chemistry 2e. Accessed 2026-07-13.
Reviewed 2026-07-13.