Empirical Formulas Versus Molecular Formulas

What are empirical formulas and how do they differ from molecular formulas?  Can a molecular formula also be an empirical formula? I will attempt to answer these questions in this post.

The empirical formula is the smallest whole number molar ratio of elements.  In other words, the numbers of atoms of each element in an empirical formula are not divisible by a common factor (other than the number one).  In molecular formulas, the number of atoms are oftentimes multiples of a common factor.  Whereas the empirical formula is the simplest ratio of two elements, the molecular formula accounts for the actual molar ratio of elements found in a particular compound. Let’s look at some examples of each.

In the above example, Compound A represents an empirical formula.  This compound has only 1 carbon atom and 2 oxygen atoms.  The quantities 1 and 2 are not divisible by a common factor (other than the number one).  On the other hand, Compound B represents a molecular formula.  There are four carbon atoms and eight oxygen atoms.  The quantities, 4 and 8, are both divisible by four (the largest common factor).  Compound B is a multiple of Compound A (multiply both elements by 4 in Compound A and you will get Compound B.)

On to the next question: Can a molecular formula be an empirical formula?  Yes it can!  The molecular formula is the formula for a compound as it exists.  For example, water (H2O) is composed of two hydrogen atoms and one oxygen atom.  The numbers, 2 and 1 are not divisible by a common factor, and therefore the formula H2O is in the lowest terms.  H2O is both the molecular formula of water (because the formula specifies the actual amounts of atoms in water) as well as an empirical formula (the molar ratio of the elements is in the lowest terms).

The formula C6H6, is the molecular formula for benzene.  However it is not an empirical formula because the numbers 6 and 6 are both divisible by the largest common factor, 6.  Dividing both elements by 6 would give an empirical formula of CH.  Note that CH is a different compound, it is not benzene.

Compound A above is also the molecular formula for carbon dioxide!

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