The molecular formula of a compound is just the number of atoms of each type in the molecule. It is always an integer multiple of the simplest formula, although that multiple is quite often 1.

To experimentally determine the molecular formula if you have the simplest formula, you need to get the molecular weight. There are a number of ways to get the molecular weight from experiment; for example, it can be determined from gas phase measurements.

Once you have the molecular weight and the simplest formula, simply divide the molecular weight by the molecular weight of the simplest formula: this will give you the integer multiple. Once you have that, simply multiply the simplest formula by this multiple and you'll get the molecular formula

Example: A molecule is analyzed and found to have a simplest formula of CH. The molecular mass of the molecule is found by gas phase measurements: it is 78.11 g/mole. What is the molecular formula of the molecule?

Solution:. The molecular weight of the simplest formula (CH) is 13.02 g/mole. We get the multiple by dividing the molecular weight by this number:

multiple = MW simplest formula/MW

multiple = 13.02/78.11

multiple = 6.00

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