# What information do you need to determine the chemical formula of an unknown compound?

Jun 4, 2017

You need its $\text{empirical formula}$, and also its $\text{molecular mass}$ to get the $\text{molecular formula}$.
The $\text{empirical formula}$ is the simplest whole number ratio that defines constituent atoms in a species. Most of the time, at least for organic compounds, this can be determined from combustion analysis, where a known mass of sample is combusted with oxygen in a furnace, and the combustion products, $C {O}_{2} \left(g\right)$, ${H}_{2} O \left(g\right)$, and $N {O}_{2} \left(g\right)$ are shunted to a gas chromatograph (sometimes $N {O}_{2}$ is reduced back to ${N}_{2}$). When these readings are compared to a standard, very accurate percentage compositions with respect to $C , H ,$ and $N$ may be measured.
These may then used to give the $\text{empirical formula}$. And then, provided that some means exist to interrogate the molecular mass of the compound is available, finally the molecular formula may be obtained in that............
$\text{molecular formula"=nxx"{empirical formula}}$........where $n$ is generally a whole number, $1$, $2$, etc.