Question

How do you find empirical and molecular formulas?

Answer 1

How else but by measurement? And two measurements must be made: (i) elemental percentage composition; and (ii) molecular mass.

Explanation:

Typically we get an unknown (organic) compound, and it contains `C,H,N,O`. We combust a known mass in a furnace, and the combustion products, `CO_2`, `H_2O`, and `NO_2` are shunted to a gas chromatograph, and the `%C, %H, %N` are delivered.

We can thus get a an empirical formula of the kind, `C_nH_mN_oO_p` (typically, oxygen is the missing percentage if `%C, %H, %N` do NOT sum to `100%`). And the empirical formula is the simplest whole number ratio that represents constituent atoms in a species. So we have the empirical formula, and we need a determination of molecular weight, before we approach the molecular formula:

i.e. `"Molecular formula "=` `nxx("Empirical formula")`, and thus we solve for `n` to get the molecular formula.

Here is one [example.](https://socratic.org/questions/saccharin-has-the-composition-45-90-c-2-75-h-26-20-o-17-50-s-and-7-65-n-what-is-)