difficulty chem question acids/base

[obogler]

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hi there,

I am having difficulty understanding #11 on the P/S section of the AAMC practice. The question is about whether the unknown molecule is a weak or strong acid. My hunch was that it was a weak acid (after all, pH 5 isn't SO basic) but then in the passage it said the molecule completely dissociates, so I picked strong.

In the solutions, AAMC writes that the concentration of the H+ ions is 10^-5... and quoting "which is high but not higher than the molecule's concentration." WHERE is that molecule's concentration ever said??

Thanks!

 

[basophilic]

From what I remember of the question, it's a weak acid because it dissolves completely but dissociates partially.

 

[BerkReviewTeach]

hi there,

I am having difficulty understanding #11 on the P/S section of the AAMC practice. The question is about whether the unknown molecule is a weak or strong acid. My hunch was that it was a weak acid (after all, pH 5 isn't SO basic) but then in the passage it said the molecule completely dissociates, so I picked strong.

In the solutions, AAMC writes that the concentration of the H+ ions is 10^-5... and quoting "which is high but not higher than the molecule's concentration." WHERE is that molecule's concentration ever said??

Thanks!

If you happen to have the BR Gen Chem book, this concept is very much like passage 2 in the chapter 4 homework set (page 270). When the compound in their question was added to solution, it (1) fully dissolved, (2) weakly conducted electricity, and (3) had an [H+] of 1 x 10^-5. This is very much like Trial 2 in the experimental table of BR passage 2.

The [H+] tells us its acidic and the low conductivity tells us that it was a neutral compound that only partially dissociates in water. The only feasible choice is a weak acid. If you have access to the BR book (or even better, the in-class handout from the lecture course), this is explained really well (along with some other common tricks).