[sun725]

How come there is such a huge difference when looking at the pKa values between these 3 compounds.

Benzoic acid: 4.2
Benzamide: 23.35
Phenol: 9-10

My answer to this and please correct me if wrong

Benzoic acid has a hydroxyl group attached to a powerfully electron withdrawing carbonyl group. This weakens the O-H bond and gives an acid which makes it more acidic than H₂O.

Phenol,  the OH is bonded to a less powerful phenyl ring. We get an acid, but weaker than the first case. 

Benzamide ... Don't know really here...

[Babcock_Hall]

With respect to benzoic acid and phenol, what about the effects of resonance?