How do you find empirical and molecular formulas?

Feb 27, 2017

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, $C {O}_{2}$, ${H}_{2} O$, and $N {O}_{2}$ are shunted to a gas chromatograph, and the %C, %H, %N are delivered.

We can thus get a an empirical formula of the kind, ${C}_{n} {H}_{m} {N}_{o} {O}_{p}$ (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. $\text{Molecular formula } =$ $n \times \left(\text{Empirical formula}\right)$, and thus we solve for $n$ to get the molecular formula.

Here is one example.