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General Chemistry I · Stoichiometry

Reaction Stoichiometry

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Reaction stoichiometry is the quantitative study of how much reactant is consumed and how much product is formed in a chemical reaction. Because a balanced equation's coefficients give the mole ratio between any two substances, a known mass of one substance can be converted to the mass of any other through a chain: grams → moles → (mole ratio) → moles → grams. The mole ratio is the single conceptual step that connects two different substances.

Why this matters

Stoichiometry is the arithmetic of chemistry: it tells a chemist how much starting material to weigh, how much product to expect, and whether a process is economical. From scaling a lab reaction to a factory, to computing how much CO₂ a car emits per liter of gasoline burned, every quantitative chemical question runs through the mole-ratio path.

The college version

Key Ideas

The mole ratio

  • Coefficients in the balanced equation give the ratio of moles of substances.
  • For 4 Fe + 3 O₂ → 2 Fe₂O₃, the Fe:Fe₂O₃ ratio is 4:2, and Fe:O₂ is 4:3.
  • A mole ratio is written as a conversion factor, e.g., (2 mol Fe₂O₃ / 4 mol Fe).

The universal four-step path

  1. Mass of known → moles of known (÷ molar mass).
  2. Moles of known → moles of unknown (× mole ratio).
  3. Moles of unknown → mass of unknown (× molar mass).

Why "grams-to-grams" is indirect

  • You can never convert grams of one substance directly to grams of another; the reaction happens in moles, so the mole ratio is the only bridge between substances.

Equations and Variables

  • Moles from mass: n = m / M
  • Mole ratio (from equation): (coefficient of target) / (coefficient of known)
  • Mass from moles: m = n × M
  • Full chain: m(A) → n(A) → n(B) → m(B)

How It Works (Problem-Solving Method)

  1. Write and balance the chemical equation.
  2. Convert the given mass to moles (divide by molar mass).
  3. Apply the mole ratio from the coefficients to find moles of the target substance.
  4. Convert moles of target to mass (multiply by its molar mass).
  5. Report with units and correct significant figures.

Worked Example

How many grams of Fe₂O₃ form when 20.0 g of Fe react with excess O₂? Equation: 4 Fe + 3 O₂ → 2 Fe₂O₃

  1. Moles of Fe: 20.0 g ÷ 55.85 g/mol = 0.358 mol Fe.
  2. Mole ratio Fe → Fe₂O₃ is 2:4, so: 0.358 mol Fe × (2 mol Fe₂O₃ / 4 mol Fe) = 0.179 mol Fe₂O₃.
  3. Mass of Fe₂O₃ (molar mass 159.7 g/mol): 0.179 mol × 159.7 g/mol = 28.6 g Fe₂O₃.

Reverse direction — mass of reactant: To make 50.0 g Fe₂O₃, how much Fe is needed?

  • 50.0 g Fe₂O₃ ÷ 159.7 g/mol = 0.313 mol Fe₂O₃.
  • 0.313 mol Fe₂O₃ × (4 mol Fe / 2 mol Fe₂O₃) = 0.626 mol Fe.
  • 0.626 mol × 55.85 g/mol = 35.0 g Fe.

Common Confusions

  • "Use the coefficients as a gram ratio." — Coefficients are a mole ratio only; grams must first be converted to moles.
  • "Convert grams of A directly to grams of B." — Impossible without the mole ratio; there is no universal grams-to-grams factor.
  • "Forgetting to balance the equation first." — The coefficients are only meaningful once the equation is balanced.
  • "Using the wrong molar mass." — Use the molar mass of the substance you are currently converting (Fe for step 1, Fe₂O₃ for step 3).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a burger recipe: 2 buns + 1 patty → 1 burger. If you have 10 buns, the recipe tells you you can make 5 burgers — the "2 buns per burger" is the recipe's ratio, and it's what lets you jump from counting buns to counting burgers. Chemistry works the same way: the balanced equation is the recipe, and the coefficients are the "per" ratios. The one extra wrinkle: you don't count atoms, you weigh them, so you first turn your grams into a count (moles), use the recipe ratio, then turn the answer back into grams.

Key takeaways

  • Stoichiometry = quantitative relationships from a balanced equation.
  • Coefficients give mole ratios, not mass ratios.
  • The path is always mass → moles → mole ratio → moles → mass.
  • Molar mass converts grams ↔ moles; the mole ratio converts between substances.
  • Never convert grams of A directly to grams of B.
  • Balance the equation; read off mole ratios.
  • grams → moles (÷ M), → mole ratio, → moles → grams (× M).
  • Mole ratio = target coefficient / known coefficient.
  • Coefficients are mole ratios, never mass ratios.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define stoichiometry and the mole ratio.
  • Convert between masses of reactants and products using the mass → moles → mole ratio → moles → mass path.
  • Identify mole ratios from a balanced equation's coefficients.
  • Carry out a multi-step stoichiometry calculation with correct units and significant figures.

Sources & references

  1. OpenStax, "4.3 Reaction Stoichiometry." *Chemistry 2e*.
  2. OpenStax, "3.1 Formula Mass and the Mole Concept." *Chemistry 2e*.
  3. OpenStax, "4.1 Writing and Balancing Chemical Equations." *Chemistry 2e*.

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