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Chemistry Lessons Review Links

Molecular Formula from %Composition & Molecular Mass

This will guide you through determining the molecular formula of a compound just based on the percent composition and molecular mass.

Molecular Formula: The actual formula for a molecule.

Problem: A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen by mass. It has a molecular weight of 318.31 g/mol. What is the molecular formula for this compound?


  1. Find the empirical formula.
    • Get the mass of each element by assuming a certain overall mass for the sample (100 g is a good mass to assume when working with percentages).
    • Convert the mass of each element to moles.
    • Find the ratio of the moles of each element.
    • Use the mole ratio to write the empirical fomula.

  2. Find the mass of the empirical unit.

  3. Figure out how many empirical units are in a molecular unit.

  4. Write the molecular formula.

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