How do we determine the molecular formula of a carbonyl species, #C_nH_(2n)O#?
Without a molecular weight determination you are a shot bird..........
Formaldehyde, acetaldehyde, acetone, all the MONOFUNCTIONAL ketones and aldehydes have a formula of
TO get the molecular formula, you need a molecular weight determination. Got one?
A useful idea to develop in this context is the so-called degree of unsaturation. You have the one degree of unsaturation for this problem.