pH titration

[cloak25]

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The addition of distilled water to which of the following solutions would increase its pH?

A. A mixture of HA and A- in aqueous solution
B. A solution of NaOH(aq)
C. A solution of NaHCO3(aq)
D. A solution of HCO2H(aq)

Someone want to explain how to answer this? I will post the correct answer later.

 

[Silo004]

D b/c you don't know pka of A and both b and c are bases i.e. high ph

 

[LoLCareerGoals]

Thanks for not posting the answer. Let me try this:

Adding water reduces concentration of existing agents. If the solution is currently acidic diluting it will make it less so by dropping [H+] and pH goes up. Opposite is true for basic. So we are now looking for an acidic solution.
A. Hmm. What element is A? Some halogen? Should be at least a weak acid either way imo.
B is obv out.
C HCO3- is more a base than an acid (Ka2 in 10^-11 range) so C is out
D H2CO2...I don't know this acid?, but it must be weaker than H2CO3 because it has 1 less oxygen, H2CO3 has a low Ka1 (10^-7ish), so I would guess this one is going to be basic.

I am a bit torn between A&D because my unfamiliarity with that compound in D, but would favor A. Even if A=F and HF is a weak acid and common ion effect should shift the balance to the left in HF-> H+ + F- still it should remain acidic, so A is my pick.
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Edit: Yeah I have to agree with poster above. If it is HF and since we don't know the relative ratios of HA and A- you could potentially saturate the solution with A- so no H+ form whatsoever making a neutral solution that isn't affected by dilution. So while I don't know what that (acid?) in D does, A is eliminatable so it has to be D. If we could restrict A to strong acid halogens (Br, I, Cl), then it wouldn't matter what the relative ratios are.
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Not to hijack, but as an MCAT noob I don't know this: Do you have to memorize all Ka values for most acids?? Did they give you these values earlier in the passage?

 

[cloak25]

Yes, the answer is D. They did not provide any Ka values. This was like a discrete, no need for the passage. Thanks for the responses!

 

[Silo004]

Just so you know, HF's pKa=3.2 formic acid's pKa=3.77 so HF is more acidic. But I still think it's a really close call. I was going to change my answer. I honestly think the most important thing about these types of questions is the thought process.

That said, this is an annoying question.

 

[LoLCareerGoals]

True but no thought process it the world will help you if you don't know that HCO3- is more basic than acidic (i.e. the PKa value). Looks like I got some appendix memorizing to do.