Final answer:
The question seems to mistakenly refer to a carbonyl cation (C=OH) while asking for a pKa value. Assuming it is a misinterpretation and the intended compound is a carboxylic acid, these typically have pKa values in the range of 2 to 5, with protons adjacent to carbonyl groups being more acidic due to resonance stabilization.
Step-by-step explanation:
The student is inquiring about the pKa value for a hypothetically written carbonyl cation (C=OH). However, there seems to be a small error in the question as written. Typically, cations of this type are referred to as acylium ions with the formula R-C+=O. Nevertheless, for the purpose of this question and given the context provided, we will refer to the pKa associated with carboxylic acids, which contain the carbonyl group (C=O), as the student might be confusing it with that.
Protons on a carbon adjacent to a carbonyl group, such as in aldehydes or ketones, are more acidic compared to protons on alkanes, with pKa values typically around 20 due to stabilization via resonance. Carboxylic acids have much lower pKa values, between 2 and 5, because of the resonance stabilization of their carboxylate ion conjugate base.
The pKa values are determined by taking the negative logarithm of the Ka value. Using Table 17.2.1 as a reference, if the student is asking about carboxylic acids, pKa values range significantly but are generally within 2 to 5. This information is essential when comparing the acidity of organic molecules.