# How would you find the molecular formula from empirical formula?

##### 1 Answer
Jan 31, 2016

You need an idea of the molecular mass.

#### Explanation:

The molecular formula is ALWAYS a multiple of the empirical formula.

Let's say we analyzed napthalene (${C}_{10} {H}_{8}$) by combustion. Microanalysis would give an empirical formula of ${C}_{5} {H}_{4}$; the empirical formula is the simplest whole number of ratio defining constituent atoms in a species (I learnt that definition years ago, and I can still recite it glibly!).

In other experiment (perhaps mass spectroscopy), we could discover that the molecular mass of napthalene was $82$ $\text{amu}$.

So, given that the molecular formula is a multiple of the empirical formula we KNOW that $M F$ $=$ ${\left(E F\right)}_{n}$.

From above $E F$ $=$ ${C}_{5} {H}_{4}$; and $M F$ $=$ $\left({C}_{5} {H}_{4}\right) \times n$, i.e. $n$ $=$ $2$.