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How Does One Determine a Molecular Formula from the Empirical Formula?

Published in Molecular Formulas 2 mins read

To determine the molecular formula from the empirical formula, you need to know the molar mass of the compound and the empirical formula.

Here's the process:

  1. Calculate the Empirical Formula Mass: Determine the molar mass of the empirical formula by adding up the atomic masses of all the atoms in the empirical formula.

  2. Determine the Ratio: Divide the molar mass of the compound (given or experimentally determined) by the empirical formula mass (calculated in step 1). This will give you a whole number (or very close to it). This whole number represents the factor by which the empirical formula must be multiplied to obtain the molecular formula.

    Ratio = (Molar Mass of Compound) / (Empirical Formula Mass)

  3. Multiply the Subscripts: Multiply all the subscripts in the empirical formula by the ratio calculated in step 2. The resulting formula is the molecular formula.

Example:

Let's say you have a compound with the empirical formula CH₂O and a molar mass of 180 g/mol.

  1. Empirical Formula Mass:

    • C: 1 x 12.01 g/mol = 12.01 g/mol
    • H: 2 x 1.01 g/mol = 2.02 g/mol
    • O: 1 x 16.00 g/mol = 16.00 g/mol
    • Total: 12.01 + 2.02 + 16.00 = 30.03 g/mol

  2. Ratio:

    • Ratio = 180 g/mol / 30.03 g/mol = 6 (approximately)

  3. Molecular Formula:

    • Multiply the subscripts in CH₂O by 6: C₁⁶H₂⁶O₁*⁶ = C₆H₁₂O₆

Therefore, the molecular formula is C₆H₁₂O₆ (Glucose).

In summary, determining the molecular formula from the empirical formula requires finding the ratio between the compound's molar mass and the empirical formula's molar mass and then multiplying the subscripts in the empirical formula by this ratio.

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