weak acid/base concept

[kazuma]

sorry I originally posted this in the wrong place;

I'm still a little rusty of this concept, please help.

If a weak acid is in an aqueous solution, it increase the pH bc the anion of the weak acid reacts with water and produces OH- molecules..?

I'm not even sure if I have the concept correct; one example states that Na2CO3 will not cause an indicator to change bc it is neutral. But isn't carbonate the anion of a weak acid (carbonic acid)?

[MANDALAM]

Kazuma, Consider the weak acid H2CO3

When it breaks in aquous solution, it gives up its first proton. The [H] of solution rises, and the pH falls as a result.

Anion of a weak acid will not react with water to give OH-. Doing so implies that the conjugate base of this weak acid is strong enough of a base that is able to pull proton off water (ie, stronger base than OH-). If it were true, the Conjugate base could simply take the dissociated proton back from the solution instead of attacking water. So, remember, the Kb of a conjugate base of a weak acid is usually lower than Ka of its conjugate acid. This is why the solution becomes acidic. If it were the other way around, it would be basic.

Hope that helped.

[kazuma]

http://dbhs.wvusd.k12.ca.us/webdocs/AcidBase/Hydrolysis.html

check that out, lemme know what you think

[kazuma]

i think i pretty much get it now, thx you rule! and good luck on the mcat!!