[pacman12]

1) Why is ortho-nitrobenzoic acid a stronger acid than para-nitrobenzoic acid?
Resonance structures. ortho-nitrobenzoic has less than para so para is more acidic.

Why are the pKa’s of meta-nitrobenzoic acid and para-nitrobenzoic acid comparable? Is this a result that you would expect? Is it because the nitro group is meta and para?

[SirRoderick]

It probably has to do with the localisation of the partial charges. I can't draw them out right now (using phone), but you'll find that in one resonance structure you probably have a positive charge on the carbon on which the nitro-group is bonded. Becasue the nitro-group has a strong inductive effect on that carbon (it draws away electrons) the positive charge becomes more pronounced and so more unstable.

The other option is that if you can manage to get a resonance structure where you remove the charge from the ring, that is far more stable seeing as how that restores the aromatic properties. Again I can't draw them out now and I'm rubbish at doing this in my mind's eye, but if you have them you should be able to see this.

Also meta and para are indeed similar. Because of the reasons above, the always have the same stabilising/destabilising effect.

Hope that made sense. 

[pacman12]

Thanks that helps a lot.
I have to come up with the resonance structures and go from there