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General Chemistry I · Stoichiometry

Molecular Formula

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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

The molecular formula gives the actual number of each type of atom in one molecule, while the empirical formula gives only the simplest ratio. The molecular formula is always a whole-number multiple of the empirical formula: molecular formula = (empirical formula) × n, where n = molar mass ÷ empirical-formula mass. To find it, you first determine the empirical formula, compute its mass, then divide the compound's measured molar mass by that empirical mass to obtain the integer n.

Why this matters

Knowing whether a substance is CH₂O or C₆H₁₂O₆ changes everything — formaldehyde is a toxic preservative, while glucose is the sugar your cells burn for energy, yet both share the same empirical formula. Distinguishing them requires the molecular formula, which is exactly what this calculation provides and why drug and materials chemists always confirm molar mass alongside elemental analysis.

The college version

Key Ideas

The relationship

  • Molecular formula = (empirical formula) × n, with n a positive integer.
  • n = (molar mass of compound) / (molar mass of empirical formula).
  • When n = 1, the two formulas are identical.

Why the molar mass is the missing piece

  • Percent composition alone gives only the ratio (empirical formula).
  • The measured molar mass tells you how many times that ratio repeats in a real molecule.

Example pair

  • Formaldehyde: molecular CH₂O, empirical CH₂O (n = 1).
  • Glucose: molecular C₆H₁₂O₆, empirical CH₂O (n = 6).

Equations and Variables

  • Multiplier: n = M(compound) / M(empirical formula)
  • Molecular formula: (empirical formula) × n
  • Empirical-formula mass: the molar mass computed from the empirical formula.

How It Works (Problem-Solving Method)

  1. Determine the empirical formula (from percent composition or masses).
  2. Compute the empirical-formula mass (molar mass of that simplest formula).
  3. Obtain the compound's molar mass (given, or measured by mass spectrometry).
  4. Calculate n = molar mass ÷ empirical-formula mass.
  5. Multiply every subscript in the empirical formula by n.
  6. Round n to a whole number — it must be an integer (small errors from rounding are corrected here).

Worked Example

A compound has the empirical formula CH₂O and a molar mass of 180.16 g/mol. Find its molecular formula.

  • Empirical-formula mass of CH₂O = 12.01 + 2(1.008) + 16.00 = 30.03 g/mol.
  • n = 180.16 g/mol ÷ 30.03 g/mol = 6.00.
  • Molecular formula = (CH₂O) × 6 = C₆H₁₂O₆ (glucose).

Full problem style: If a compound is 92.3% C and 7.7% H by mass with molar mass 78.1 g/mol:

  • Empirical: 92.3 g C ÷ 12.01 = 7.69 mol; 7.7 g H ÷ 1.008 = 7.64 mol; ratio ≈ 1 : 1 → CH.
  • Empirical mass of CH = 13.02 g/mol. n = 78.1 ÷ 13.02 = 6.00.
  • Molecular formula = C₆H₆ (benzene).

Common Confusions

  • "The empirical formula is always the molecular formula." — Only when n = 1; many compounds (glucose, benzene, acetic acid) have n > 1.
  • "n can be a fraction." — No; you cannot have half a molecule. If n computes to 1.97, round to 2.
  • "Compute n with the molecular mass in the denominator." — The divisor is the empirical mass; n = molar mass / empirical mass.
  • "Empirical and molecular formulas have the same mass." — They do only when n = 1; otherwise the molecular mass is n times larger.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The empirical formula is like the "serving size" of a recipe, and the molecular formula is the whole batch. If your serving is 1 cup flour : 2 sugar : 1 egg, but you actually baked a batch six times that size, the real batch is 6 : 12 : 6. Chemists find out how big the batch is by weighing the whole thing: the batch's total weight divided by one serving's weight tells you the multiplier (6). The catch: this only works because every serving weighs exactly the same — for molecules, "weighing the batch" means measuring the molar mass, which tells you how many empirical units are strung into one real molecule.

Key takeaways

  • Molecular formula = empirical formula × n.
  • n = molar mass ÷ empirical-formula mass.
  • n must be a whole number (round to the nearest integer).
  • n = 1 means molecular and empirical formulas are identical.
  • The empirical-formula mass is computed from the empirical formula, not the molecular formula.
  • Find the empirical formula first.
  • n = molar mass ÷ empirical-formula mass.
  • Multiply all empirical subscripts by n.
  • Round n to a whole number.

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 molecular formula and relate it to empirical formula.
  • Calculate the multiplier n = molar mass / empirical-formula mass.
  • Determine a molecular formula given the empirical formula and the molar mass.
  • Explain when the molecular formula equals the empirical formula.

Sources & references

  1. OpenStax, "3.2 Determining Empirical and Molecular Formulas." *Chemistry 2e*.
  2. Chemistry LibreTexts, "10.12: Determining Empirical Formulas."
  3. Chemistry LibreTexts, "10: The Mole."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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