What does an empirical formula determine?

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, $N {O}_{2}$, and ${N}_{2} {O}_{4}$. Clearly, these molecules both possess an empirical formula of $N {O}_{2}$ (why?). But because of the lone electron on $N {O}_{2}$, the molecule can dimerize according to the following equation:

$2 N {O}_{2} \left(g\right) r i g h t \le f t h a r p \infty n s {N}_{2} {O}_{4} \left(g\right)$

i.e. ${\text{^(-)O(O=)}}^{+} N \cdot + \cdot {N}^{+} \left(= O\right) {O}^{-} r i g h t \le f t h a r p \infty n s$ ${\text{^(-)O(O=)}}^{+} N - {N}^{+} \left(= O\right) {O}^{-}$.

To approach the molecular formula from the empirical formula, we need an estimate of the molecular mass of the molecule in $g \cdot m o {l}^{-} 1$.

i.e. $\text{Molecular formula}$ $=$ $\text{(empirical formula)} \times n$

$n$ is a whole number to be determined.