Empirical vs. Molecular Formulas

Overview


When going through introductory chemistry courses, a common problem that you will run into is your professor asking for either empirical or molecular formulas. This can be a bit confusing when you first run into it, so I’m going to clear up the difference between the two, and when to use one or the other if not instructed specifically.


Molecular Formula


The molecular formula is the chemical formula for molecules that includes the integer amount of each atom you determined from your calculations, or that were given to you. When writing a formula, you include the integer amounts of the atoms in the subscript. An example is included below for a molecule with 6 atoms of Carbon, 12 atoms of Hydrogen, and 6 atoms of Oxygen.

The molecular formula of glucose

Empirical Formula


The empirical formula for molecules is the chemical formula that is the simplest form of the molecular formula. This can be found by dividing all the subscripts in the formula by their LCD (Lowest Common Denominator). An example of the empirical formula for the previous example is given below.



The empirical formula of glucose

Deciding Which to Use


Both the empirical and molecular formula can be useful depending on the circumstances. The empirical formula is generally used to simply show what elements are present in a molecule. This is useful when one wants to know at a glance what elements they are dealing with.


The molecular formula is most useful when you wish to know how many atoms of the elements are present in the compound. It gives more information than the empirical formula, and is therefore more common. The molecular formula is especially important when you start to work with organic chemistry.


Closing Thoughts


The most important thing to remember when working with molecular and empirical formulas is to divide the subscripts of the molecular formula to obtain the empirical formula. Pay close attention to what the questions, or your professor are asking for, and practice, practice, practice.

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