Simple pKa question

[Machine33]

Long story short, a large pka is indicative of a weaker base, right? I am just asking to confirm TBR made a mistake in one of the questions in their book and I didn't miss anything.

"Which of the following acids has a LARGER pKa value?

A. HClO2
B. HIO2
Other 2 are irrelevant =)

B is supposedly the correct answer and in the solution it states that the question could have asked just as easily for the strongest acid.

thanks for your help!

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[MT Headed]

I will vouch for the fact that the phrase "LARGER pKa value" means "weakest acid" and not "strongest acid".

I am uncomfortable with the rest of the question. HIO2 seems to be a theoretical molecule that has never been isolated. In terms of acid strength certainly HF < HCl < HBr < HI, but when you start adding oxygens some of the rules seem to get twisted. According to wikipedia, HBrO3 < HClO3 < HIO3 for instance.

 

[Machine33]

So would you say just don't pay it any attention?

 

[MT Headed]

So would you say just don't pay it any attention?

👍

But I did notice that in your OP you said large pKa = weaker base. I don't like that. pKa refers to acid strength, let's leave it at that. larger pKa = weaker acid. It would be difficult to make conclusions about the basicity of a molecule given its pKa, because basicity refers to its ability to take on (steal) another proton.

 

[Machine33]

Oops, yeah I meant to put acid lol. Nice catch, you must be a pro at verbal!

 

[chiddler]

It does warrant attention. TBR writes a bit in the acid chapter about oxyacids. This question is asking you to be familiar with the trend.

1. More oxygens = more acidic because more pi bonds means more resonance stabilization and a more stable conjugate base.

2. More electronegative central atom = more acidic because more pull from oxygens allows stabilization through inductive effect.

 

[MT Headed]

So to answer this question, they have the same number of oxygens so rule 1 does not apply. Cl is more electronegative than I, so rule 2 says the HIO2 has a larger pKa (weaker acid).

Interesting that rule 2 only applies when oxygens are present. If there were no oxygen, HCl is weaker than HI.

 

[MedPR]

TBR is correct. HIO2 is LESS acidic (therefore it has a HIGHER pKa) than HClO2. Cl is more electronegative than I, and makes HClO2 the stronger acid.

Since the halogens are the central atoms, the one that withdraws electron density more will produce the stronger acid.

It does warrant attention. TBR writes a bit in the acid chapter about oxyacids. This question is asking you to be familiar with the trend.

1. More oxygens = more acidic because more pi bonds means more resonance stabilization and a more stable conjugate base.

2. More electronegative central atom = more acidic because more pull from oxygens allows stabilization through inductive effect.

Just to add, rule 2 applies only when you are comparing acids with the same number of oxygens. I'm sure the trend gets muddled quite a bit, but, for the MCAT, when you have more oxygens, the central atom is irrelevant, unless values are given.

And, as far as the part about "could have easily asked about the strongest acid" goes, they are just telling you (as they did in the chapter) that the key to a lot of these questions is recognizing that a lot of the time they are asking "what is the strongest acid? weakest acid? strongest base? weakest base?" in some round about way. They are not saying that the highest pKa means the strongest acid.

 

[Machine33]

That makes perfect sense. Thanks for all your replies!