pKa2 for H2SO4

[theonlytycrane]

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The answer is (C) I and II only. Should HSO4- be considered weakly acidic? I know that H2SO4 fully dissociates, but I wasn't sure how to think of HSO4- for MCAT purposes.

 

[betterfuture]

Well at dilute concentrations of H2SO4, the second ionization can't be ignored. So in dilute concentrations, the HSO4 would contribute to the pH(as an acid). But since they did not say the solution is dilute, the second ionization is negligible and acts as a weak base. However, it won't decrease the pH. Are you sure II is included as part of the answer?

To me, it seems only I is the right answer.

 

[theonlytycrane]

@betterfuture yeah, the answer given was (C). But I chose only I as I agree with you!

 

[salemstein]

Without giving a pKa table, this would be a bad question. I actually Googled the pKa2 of H2SO4 and its 1.99 (pKa1 = -10). I guess this is simply an exception where the original is so acidic that even its conjugate is still acidic. The only other "tricky" polyprotic acid on the list that violates the rule was citric acid. Tho its a weak acid, it still remains acidic even after losing proton #2 as well as #3.

 

[popopopop]

Why would HSO4- not contribute to acidity? It certainly wouldn't act as a bronsted base by becoming H2SO4. I understand it's the conjugate base of H2SO4, but HSO4- should continue to add H+ to the solution.

*edit* I miss aldol16 in this section, so educational lol. Why am I even here, I took the MCAT.

 

[theonlytycrane]

@popopopop I thought it was ambiguous because HSO4- would be weakly acidic and only slightly decrease pH, if at all.

 

[betterfuture]

Why would HSO4- not contribute to acidity? It certainly wouldn't act as a bronsted base by becoming H2SO4. I understand it's the conjugate base of H2SO4, but HSO4- should continue to add H+ to the solution.

For polyprotic acids, usually the first Ka1 acid ionization constant is bigger than the other acid ionization constants. This is because the first proton separates from a neutral molecule while the second proton would separate from the anion, a negatively charged species. The anion will decrease the likelihood of the proton deprotonating since it is attracted to the positively charged H+.

 

[popopopop]

Well, the definition of acid salt clears it up a bit if you trust the source. I vaguely remember these types of question in gen chem 2.

https://en.wikipedia.org/wiki/Acid_salt

 

[popopopop]

For polyprotic acids, usually the first Ka1 acid ionization constant is bigger than the other acid ionization constants. This is because the first proton separates from a neutral molecule while the second proton would separate from the anion, a negatively charged species. The anion will decrease the likelihood of the proton deprotonating since it is attracted to the positively charged H+.

Ahh, that's a good way of explaining it, but I'm approaching it a bit less technically. I get your logic. I just think in my head H2SO4 completely dissociates, sulfates are usually soluble, so I want those H+ and sulfates floating around if they are in a solution.

 

[7331poas]

Even just eyeballing it you know h2so4 is far more acidic than hydronium, it would follow that their conjugate bases will follow that same pka trend.