Question

What does an empirical formula determine?

Answer 1

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.