# What information must a chemist have to determine the molecular formula for a compound?

May 31, 2018

Well, for a start the chemist must have the "empirical formula" ...

#### Explanation:

And what is the $\text{empirical formula}$...? It is the simplest WHOLE NUMBER ratio defining constituent atoms in a species. And thus the $\text{empirical formula}$ for $C {H}_{4}$, ${C}_{2} {H}_{6}$, and ${C}_{3} {H}_{8}$, are $C {H}_{4}$, $C {H}_{3}$, and ${C}_{3} {H}_{8}$ respectively...With me? Combustion analysis of organic compounds is nowadays routine....$\text{% by mass}$ with respect to carbon, hydrogen, nitrogen, may be readily acquired...and this allows determination of the empirical formula..

And then some determination of molecular mass is obtained…the $\text{molecular formula}$ is a whole number MULTIPLE of the $\text{empirical formula...}$...i.e.

$\text{molecular formula"-=nxx"empirical formula}$

And of course $n$ $\text{MAY EQUAL}$ $1$..

See here for a representative problem....