Why would a carboxylate anion be more soluble in water than a carboxylic acid? Wouldn't the hydrogen bonding of a carboxylic acid be stronger than the polar effect of the anion?

What bonding forces are present in benzoic acid? How does benzoic acid interact with water?

 

What bonding forces are present in sodium benzoate? How does the benzoate interact with water?

 

Keep in mind that not all carboxylic acids are alike. Lower molecular weight carboxylic acids such as acetic acid are soluble in water. This solubility begins to decrease drastically after C5. Why would such a pattern exist? What can you say about the carbon/heteroatom ratio in acetic acid? In benzoic acid?

 

Finally, benzoic acid is a solid. What crystal packing forces are involved in such a solid? Why would you expect water to disrupt such crystal packing forces?

 

Good luck!

although carboxylic acid has the ability to form hydrogen bonding, however carboxylic anion will exist as an ionic salt, and ionic salts are highly stable due to partial charge on oxygen and hydrogen of water.

the carboxylate anion is more soluble because it is aleady an ion (ionised)

Carboxylic acids can form intermolecular hydrogen bond dimers with eachother that are held together quite strongly by resonance assisted hydrogen bonds. If you deprotonate and make an anion this can no longer happen. That's one contribution. But in general converting organic compounds to salts will make them more soluble in water.

Empirically aromatic carboxylic acids (benzoic acid and aspirin, for example) are not very soluble in water but are quite soluble in diethyl ether. Their conjugate base forms are quite soluble in water. This difference has a good deal of utility when trying to separate one molecule from another by extraction.