What does an empirical formula determine?

1 Answer
anor277
Oct 10, 2016

The empirical formula is the simplest whole ratio that defines the proportions of elements in a species.

Explanation:

The molecular formula is always a mulitple of the empirical formula, and of course the multiple might be #1#.

To illustrate this, the best real compounds to consider are the nitrogen oxides, #NO_2#, and #N_2O_4#. Clearly, these molecules both possess an empirical formula of #NO_2# (why?). But because of the lone electron on #NO_2#, the molecule can dimerize according to the following equation:

#2NO_2(g) rightleftharpoonsN_2O_4(g)#

i.e. #""^(-)O(O=)""^(+)N* + *N^(+)(=O)O^(-) rightleftharpoons# #""^(-)O(O=)""^(+)N-N^(+)(=O)O^(-)#.

To approach the molecular formula from the empirical formula, we need an estimate of the molecular mass of the molecule in #g*mol^-1#.

i.e. #"Molecular formula"# #=# #"(empirical formula)"xxn#

#n# is a whole number to be determined.

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