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Chemistry Lessons Review Links

Molecular Formula from %Composition & Molecular Mass

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?

Strategy:

  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).

      (.7546) (100 g) = 75.46 g C

      (.0443) (100 g) = 4.43 g H

      (.2010) (100 g) = 20.10 g O

    • Convert the mass of each element to moles.

      (75.46 g C) (1 mol/ 12.00 g C) = 6.289 mol C

      (4.43 g H) (1 mol/ 1.008 g H) = 4.39 mol H

      (20.10 g O) (1 mol/ 16.00 g O) = 1.256 mol O

    • Find the ratio of the moles of each element.

      (1.256 mol O)/ (1.256) = 1 mol O

      (6.289 mol C)/ (1.256) = 5.007 mol C

      (4.39 mol H)/ (1.256) = 3.50 mol H

    • Use the mole ratio to write the empirical fomula.

      Multiplying the mole ratios by two to get whole number, the empirical formula becomes:

      C10H7O2

  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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