AAMC4 PS Question 23

[ap594]

The question refers to the pH at the equivalence point of the titration of benzoic acid with sodium hydroxide. I got the question right; the pH would be above 7. I always learned that the pH of a titration of a weak acid would be higher than 7 because at pH=7, there is still some hydrogen ions attached to the weak acid (since it does not entirely dissociate). The explanation, though, says that the pH is higher than 7 because "benzoate ion is a stronger base than OH-", etc. etc. Why do they say that benzoate ion is a stronger base? If its conjugate acid's pKa is 4.something and water's is 7, wouldn't OH- still be the stronger base because its conjugate acid is weaker? I think I am not entirely confident in my understanding of why there are still hydrogen ions attached to benzoate when there is OH- in solution.

Thanks for the help.

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[Phantastic]

At the equivalence point you are left with:

Assuming 1 M NaOH used against 1 M Benzoic Acid --->

1 M Na+ (stable, doesn't react w/ water to change pH)
1 M Benzoate Anion (conjugate base of benzoic acid, some will react with water to produce OH-, which will raise pH slightly)
1 M H2O (negligible, produced in reaction and added to [solvent], which was already ~55.5 M)

I think this all you need to think of on the MCAT...

 

[RogueUnicorn]

the reason the pH is above 7 is this. if you have 10 benzoic acids to which you add 10 NaOH, you are at the equivalence point. now you have 10 water molecules and 10 benzoates. benzoate is a base, and will take a proton from water, creating a basic solution.

how did the benzoate rip a proton from water? by being a stronger base than OH-, of course, or else the OH- will take the proton right back. as you can see by now, pretty much any base in an aqueous solution must be stronger base than OH, or else it could not raise the pH of the solution.

did that make sense? the actual reason benzoate is a stronger base than OH i imagine is mainly related to the resonance stabilization of the O- across the carboxyl group

 

[RogueUnicorn]

the benzoate carboxylate resonance stabilization doesn't actually involve the phenyl group. draw it out

 

[ap594]

If benzoate ion exhibits resonance stabilization because of its carboxyl group, that would make the solution more acidic because there's less charge on the oxygen to maintain the bond with H+... right? I feel like saying that benzoate is a stronger base than OH- goes against basic organic chemistry. If benzoic acid is a stronger acid than water (which it clearly is, since its pKa is 4.something) then wouldn't its conj. base be weaker than OH-? Where is my logic going wrong? lol My understanding was only: benzoate is a base, and thus, at the equivalence point, some of the benzoate ion pulls an H+ off of water and produces a basic solution.

 

[RogueUnicorn]

If benzoate ion exhibits resonance stabilization because of its carboxyl group, that would make the solution more acidic because there's less charge on the oxygen to maintain the bond with H+... right? I feel like saying that benzoate is a stronger base than OH- goes against basic organic chemistry. If benzoic acid is a stronger acid than water (which it clearly is, since its pKa is 4.something) then wouldn't its conj. base be weaker than OH-? Where is my logic going wrong? lol My understanding was only: benzoate is a base, and thus, at the equivalence point, some of the benzoate ion pulls an H+ off of water and produces a basic solution.

this is correct. the rest of it is wrong i think

 

[phltz]

The explanation provided by AAMC is incorrect. Benzoate is a _weak_ base, it is _not_ stronger than OH-. You can look this information up on Wikipedia. Benzoic acid has a pKa of 4.21, which means benzoate has a pKb of 9.79. This is nowhere near as strong as OH-, which has a pKb of 0.

Even weak bases can deprotonate water to a certain extent. If you put a bunch of sodium benzoate in water (which is essentially what we're doing in this problem), it is not thermodynamically favorable for a molecule of benzoate to take a proton from a molecule of water. This is why only a small handful of the sodium benzoate molecules will do so. This is why it's known as a "weak base".

Nonetheless, a small handful of water molecules get deprotonated, so the solution is overall basic, with a pH above 7.

 

[ap594]

Thanks dude. That was my understanding, too; you posted as I was reading the responses. Appreciate it.