What's the pKa (range) of a carbonyl functional group?
As per comments you are asking for the $\mathrm{p}K_\mathrm{a}$ value of a hydrogen directly attached to a carbonyl carbon. This means, you are asking only for non-enolisable aldehydes as ketones have no hydrogens on the carbonyl carbon. Aldehydes that have a hydrogen in &#x3b1;-position will be deprotonated there generating an enolate ($\mathrm{p}K_\mathrm{a} \approx 20$) — more acidic than a carbonyl hydrogen. So all we have left is 2,2-dimethylpropanal (or pivaloylaldehyde). Its $\mathrm{p}K_\mathrm{a}$ is not given on SciFinder, but should be somewhere between $45$ (an alkane) and $25$ (an alkyne), probably just slightly higher than an alkene ($40$).