CALCULATIONS BASED ON FORMULAE

7.9 - From Empirical Formula to Molecular Formula

In section 7.8 we learned that the empirical formula of a compound can be determined from mass data. The empirical formula derived from mass data may or may not be the molecular formula of the compound. In other words,

Is the empirical formula of a compound the same as the molecular formula?
Click here to find out for sure.

The molecular formula for a compound gives the actual number of atoms of each element present in a molecule. Therefore, in order to determine the molecular formula from a compound's empirical formula, the molar mass of the compound must be known.

From the molar mass and the empirical formula, we can determine the number of empirical formula unit.

The molecular formula can be determined by multiplying each subscript in the empirical formula by the number of empirical formula units. Let's take a look at how to determine the molecular formula in the example below:

Example

The molar mass of a compound containing only carbon and hydrogen has a molar mass of 114.0 g/mole. Further analysis determined the empirical formula of the compound to be C₄H₉. What is the molecular formula for this compound?

Section 8.5
Calculation of Empirical (Simplest) and Molecular Formulas ..p200

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