[cheukyin]

Could you tell me the explaination of : "ethanoic acid is very soluble in water,
while benzoic acid is only slightly soluble in water"
Is this related to the delocalization of electron in the benzoic acid?

[lavoisier]

Predicting the solubility of a compound in water (or in any solvent for that matters) is a very complex problem. A single molecular property of your compound is generally not enough.

In this specific case I guess the delocalisation doesn't count much. They are acids of similar strength, benzoic acid being slightly stronger.
But the reason why it's not soluble is more related to the presence of the benzene ring. In its unionised form (PhCOOH) it is too 'lipohylic', i.e. it likes more to stick with other PhCOOH than to mingle with water.
If you make the salt, though, say PhCOONa, it's quite soluble in water.

As usual I apologize to the physical chemists out there for my naif, organic approach to this serious problem.