Question

How would you find the molecular formula from empirical formula?

Answer 1

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_10H_8`) by combustion. Microanalysis would give an empirical formula of `C_5H_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` `"amu"`.

So, given that the molecular formula is a multiple of the empirical formula we KNOW that `MF` `=` `(EF)_n`.

From above `EF` `=` `C_5H_4`; and `MF` `=` `(C_5H_4)xxn`, i.e. `n` `=` `2`.