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Stoichiometry Calculator

Calculate molar relationships and quantities in chemical reactions

Stoichiometry Calculator Guide

A stoichiometry calculator balances chemical equations, finds the limiting reagent, and converts between grams, moles, and molecules so you can predict theoretical yield and percent yield. It uses molar masses derived from standard atomic weights and the balanced equation coefficients.

What is Stoichiometry Calculator?

The stoichiometry calculator takes a chemical equation (e.g., C3H8 + O2 -> CO2 + H2O), balances it, then uses mole ratios to interconvert reactants and products. It supports hydrates (e.g., CuSO4*5H2O), polyatomic ions, and multi-step pathways if you chain calculations.

How to Use the Stoichiometry Calculator

  1. Enter the unbalanced reaction formula.
  2. Balance the equation; note the integer coefficients.
  3. Enter known quantities for one or more reactants (grams or moles).
  4. Calculate the limiting reagent, theoretical product amounts, and excess reagent remaining.
  5. (Optional) input actual yield to compute percent yield.

Formulas & Methods

  • Moles: n = m / M where m is mass (g) and M is molar mass (g/mol).
  • Mole ratios: from balanced coefficients aA + bB -> cC + dD, n_C = (c/a)*n_A etc.
  • Limiting reagent: compare available moles divided by their coefficients; the smallest ratio limits.
  • Theoretical yield: product moles from limiting reagent times molar mass.
  • Percent yield: = (actual / theoretical) * 100%.
  • Gas at STP (optional): V = n*22.414 L (older) or 22.711 L (IUPAC), specify standard used.

Assumptions & limitations

  • Molar masses use average atomic weights; isotopic enrichment will differ.
  • STP conventions vary; the tool defaults to IUPAC 0 C, 1 bar if selected.
  • Activity coefficients and non-ideal behavior are not modeled; this is a mass-balance tool.

Examples

Example A — Propane combustion
Unbalanced: C3H8 + O2 -> CO2 + H2O.
Balanced: C3H8 + 5O2 -> 3CO2 + 4H2O.
Given 44.0 g C3H8 (n = 1.0 mol), oxygen in excess.
n_CO2 = 3*1.0 = 3.0 mol → m = 3.0*44.01 = 132.0 g.
n_H2O = 4*1.0 = 4.0 mol → m = 4.0*18.015 = 72.06 g.

Example B — Limiting reagent
2Al + 3Cl2 -> 2AlCl3. Suppose m_Al = 5.40 g (0.200 mol), m_Cl2 = 14.2 g (0.200 mol).
Needed ratio Al:Cl2 = 2:3. Compare 0.200/2 = 0.100 vs 0.200/3 = 0.0667 → Cl2 limiting.
Product AlCl3 moles = 2*(0.0667) = 0.133 mol → mass = 0.133*133.34 ~ 17.7 g.

| Step | Formula | Result | |---|---|---| | Balance | Coefficients | C3H8 + 5O2 -> 3CO2 + 4H2O | | Moles | n = m/M | 44 g / 44.10 ~ 1.0 mol | | Yield | m = n*M | CO2 ~ 132 g |

Pro Tips & Best Practices

  • Always balance before any mass calculations; wrong coefficients break everything.
  • Convert all masses to moles first; do not mix grams directly with coefficients.
  • Track significant figures from inputs; avoid rounding until the end.
  • For solutions, use molarity (mol/L) and volume to get moles: n = C*V.
  • For gases, state the STP definition used (22.414 vs 22.711 L/mol).

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FAQ

Q: How do I balance a chemical equation?

A: Adjust coefficients so atoms of each element are equal on both sides; the calculator can suggest minimal integer coefficients.

Q: What is a limiting reagent?

A: The reactant that runs out first; it limits how much product can form in the reaction.

Q: How do I convert grams to moles?

A: Use moles = mass (g) / molar mass (g/mol); get molar mass from the chemical formula.

Q: How do percent yield and theoretical yield differ?

A: Theoretical yield is the maximum possible; percent yield = actual/theoretical x 100%.

Q: Do I need molar masses for hydrates and ions?

A: Yes—include waters of hydration and ionic charges do not affect molar mass.

Engineering note: Uses average atomic weights and stoichiometric coefficients from balanced equations; assumes ideal mixture behavior.

Call to Action

Enter your reaction, balance it, and input known quantities—the calculator will identify the limiting reagent and compute yields step by step.