Determining correct pKas in organic chemistry

[future_vet]

Homework Statement

This is a problem taken from an old exam:
C. Which set of pKa’s is correct?
1. CH3COCH3 -7.0 2. CH3COCH3 -19.0
CH3COOH 4.8 CH3COOH -4.8
HCl 19.0 HCl 7.0

3. CH3COCH3 4.8 4. CH3COCH3 19.0
CH3COOH 7.0 CH3COOH 4.8
HCl 16.0 HCl -7.0

Homework Equations

None that I know of.

The Attempt at a Solution

I know from looking at the pKa values we were given in the lecture than the right answer is #4. Nevertheless, I was wondering if there was a way to get to this answer without learning the pKas by heart (a reasoning that would make the question logical).

Thank you,

Joanna.

 

[GCT]

HCl has to have a pKa that is negative, tell me why.

 

[future_vet]

The higher the pKa, the more basic a substance is. HCl is a strong acid, therefore it should be negative... correct?

 

[GCT]

Yep, tweak the pKa equation and see for yourself.

 

[Decan]

A general rule that I learned in Ochem...know water (about 16)...strong bases will have a high pKa and a strong acids will have a low one. Weak acids will be close to water.