# How would you determine the molecular formula of C4H9?

${C}_{4} {H}_{9}$ is not the formula of anything you could put in a bottle. It is the radical of some butane derivative, i.e. ${C}_{4} {H}_{10}$..
And so we have (possibly) $\text{tert-butyl radical}$...${\left({H}_{3} C\right)}_{3} C \cdot$, the which is relatively long-lived...
On the other hand, you MIGHT have quoted an empirical formula of ${C}_{4} {H}_{9}$...to get the molecular formula, we need a measurement of the molecular mass...say we got a mass of $114.22 \cdot g \cdot m o {l}^{-} 1$...this would correspond to a molecular formula of ${C}_{8} {H}_{18}$...and this is a saturated formula...