How to calculate molecular formula?

Calculating the molecular formula involves determining the actual number of atoms of each element present in a molecule. This is different from the empirical formula, which represents the simplest whole-number ratio of atoms in a compound. Here's how to do it:

Steps to Calculate the Molecular Formula

1. Determine the empirical formula. This formula gives the simplest whole-number ratio of atoms in the compound. You might obtain this from experimental data, such as combustion analysis.

2. Calculate the empirical formula mass. Add up the atomic masses of all the atoms in the empirical formula.

3. Determine the molar mass of the compound. This value is usually provided in the problem or can be determined experimentally (e.g., using mass spectrometry).

4. Calculate the ratio (n) between the molar mass and the empirical formula mass. This step is crucial.

   • Divide the molar mass of the compound by the empirical formula mass.
   • n = (Molar Mass) / (Empirical Formula Mass)
   • The result should be a whole number or very close to a whole number, according to 6.9: Calculating Molecular Formulas for Compounds.

5. Multiply the subscripts in the empirical formula by n. This gives you the molecular formula.

Example

Let's say a compound has an empirical formula of CH₂O and a molar mass of 180 g/mol.

1. Empirical Formula: CH₂O

2. Empirical Formula Mass: 12.01 (C) + 2(1.01) (H) + 16.00 (O) = 30.03 g/mol

3. Molar Mass: 180 g/mol (Given)

4. Calculate n: n = (180 g/mol) / (30.03 g/mol) ≈ 6

5. Molecular Formula: Multiply the subscripts in CH₂O by 6: C₆H₁₂O₆

Therefore, the molecular formula is C₆H₁₂O₆.