Chad's rule for Acidic

RCT PC CRN said:

Chad says CARDIO - Charge, Atoms, Resonance, Dipole Induction and Orbitals

I compared following compounds using above rule and I got it wrong. Chad did mention of few exceptions to the rule. Is this one of the exception? or I have understood the rule wrong?

Organic Chemistry Odyssey, Ch 2, Q#1, P.23

Which compound has the smalles pKa? (i.e. strongest acid, weaker conj. base or more stable conj base..blah..blah..)

1. CH2SO3H
2. Cyclopentanethiol, -SH group attached to Cyclic pentane.
3. 1,3-dioxo-cyclohexane - 2 ketone attached to cyclohexane at B-position to each other
4. 2-pentanone
5. H3PO4

First I compared Charge.
1. O-
2. S-
3. C-
4. C-
5. O-

All compound has 1 negative charge. So I moved to second which is comparing charge on Atom.

As per the rule negative charge on S is more stable or less basic (going down the VI-A column). Similar to going down at halo group.

So I stopped here and picked 2. Cyclopentanethiol as my answer. But the correct answer is 1. CH3SO3H.

I understand -SO3H has more resonance but as per the rule you don't even get to resonance cause you can simply compare the charge on atom here. Have I understood the rule incorrectly? Am I missing anything? or I have made mistake in getting conj base charge? If someone can explain to me in detail, please.

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Cool Beans said:

This is one of the exceptions. Other notable exceptions to these rules include that strong acids like HCl and carboxylic acids are stronger than ammonium ions (RNH3+) even though ammonium ions have a plus charge. And terminal alkynes are more acidic than ammonia even though carbon is less electronegative. No set of rules is full-proof but this is as a good a set as is out there.

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