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AAMC Unscored #11
- Thread starter wheatthinners
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I'm not sure about the weakly conducting part but a strong acid can still produce a pH of 5 in water depending on how much water it is added to...that has nothing to do with the strength of the acid. I too am confused when it says that it is "much less than the concentration of the unknown compound itself" when nowhere in the passage do we know this information. I guess I was overthinking it and should've accepted that most times something with a pH of 5 will be a weak acid and the AAMC is looking for general knowledge like that not necessarily anything more specific, but it bugs me that it could technically be a strong acid.
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It could be a strong acid only if you're working at the extremes, i.e. adding the acid to a lot of water. What you're claiming is that pH has nothing to do with the strength of an acid and that's not entirely true. Given, when we teach it in gen chem, we simplify things, but not to a great extent.
The fact that the solution is weakly conducting tells you that it only weakly ionizes in water such that only a little dissociates. In other words, the Ka is low and the pKa is high. Of course, this assumes that it's not in a solution with common ions, etc., but when we think about a case, we don't think about the extremes.
You are completely correct in that the assumptions are not clearly stated and thus it is difficult to find a correct solution for this problem. But I think that's what happens when you have mainly biological people writing such questions.
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I don't understand why it is considered a weak acid. If it dissolves completely in water doesn't that mean it's as strong acid? It says that it's less than the concentration of the unknown compound itself. How do you know that?
@wheatthinners
They tell you the [H+] = 1 x 10^-5, which means pH = 5. Clue 1 that this is a weak acid.
Next they say this solution is weakly conducting, meaning is dissolved in water yet released few ions into the solution by which electricity could conduct. Clue 2 this is a weakly dissociating, molecule, and the pH < 7 indicates an acid.
Regardless of the original molecule's actual identity, what you have is a weakly acidic solution.
Hope this helps, good luck!
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Exactly. They never actually stated that in the passage or question, and without that piece of information you cannot decide between C and D.
The conditions of pH = 5 and weak conductivity could be attained a few different ways:
1) 0.000010 M HCl would generate 1 x 10-5 M H+ and 1 x 10-5 M Cl-, which would have a pH = 5 and a low enough ion concentration that electricity would not conduct.
2) 0.003 M HOCl would generate 1 x 10-5 M H+ and 1 x 10-5 M ClO-, which would have a pH = 5 and a low enough ion concentration that electricity would not conduct.
You make an excellent point here. The reality is that a 0.000010 M strong acid would have a pH = 5.0 and would be weakly conducting (due to so few ions in the water). In their answer explanation, the AAMC people state that "the hydrogen ion concentration is greater than 1.0 x 10-7 but much less that the concentration of the unknown compound itself." Unfortunately, nowhere in the passage or question do they actually tell you the concentration or quantity of the unknown compound. So technically, you cannot decide between C and D. But I agree that you can 'feel' that they want you to pick choice C.
Hopefully they edit questions better on the actual MCAT.
