AAMC Sample C/P Q11

[poochimaster]

Okay so I picked D because of the second last sentence in the passage that states: the unknown compound completely dissolves in water. Is it not true that acids that completely dissolve in water are considered strong? Also I don't get the explanation; nowhere in the passage does it state what the starting concentration of unknown compound is.

Thanks!

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[laczlacylaci]

There are actually 2 pieces of information that leads to weak than strong.
1. second line has "buffering agent" this automatically rules out B & D. A buffering agent is only a weak acid or weak base.
2. In the second to last line it says the solution is a weakly conducted electricity, meaning it is a weak acid or base, not strong. Strong acids or bases are strong conductors. (review electrolyte/solubility)
3. Since in the passage it says the [H+]=1E-5, then [OH-]=1E-9 because they need to add up to Kw=1E-14
Since [OH-]<[H+], you know this is going to an acidic solution
You are left to choose D.

If you only read the last sentence to infer to D=strong acid, how did you get acid vs. base?

It is also important to understand the difference between dissolve and ionize
A weak acid (ie. vinegar) dissolves in water, but does not completely (100%) ionize.
A strong acid/base does dissolve in water, and does completely ionize.
Pretty much anything will dissolve in water if you give it enough time.
Here is a visual image:

Notice how the weak acid does not completely ionize into H+ and A- like it did in the strong acid.

 

[NextStepTutor_3]

Yes, it's the dissolve vs. ionize distinction that is important here. (Take a look at whatever content books you've used for acid/base chemistry - it's highly unlikely that they ever say that strong acids "completely dissolve" in water. Instead, they probably use the term "ionize" or "dissociate.")

The previous answer was really good, but just to elaborate with an example - consider NaH2PO4 (monosodium phosphate). This is a sodium salt, so it will certainly dissolve into Na+ and H2PO4-. In fact, if you remember your solubility rules, you'll know that this compound is probably highly soluble in water. However, just the fact that NaH2PO4 dissolves tells us nothing about the strong vs. weak status of H2PO4. In fact, H2PO4 hardly ionizes/dissociates into H+ and HPO4 2- at all, meaning that it is a weak acid.

 

[poochimaster]

Thank you for the amazing responses guys! Everything is so clear now .

 

[LnSin]

Maybe I'm being foolish but don't they tell you the pH? Since [H+]=1E-5, pH is 5, so you instantly know it's acidic. Since the pH is greater than, say, 2 you know it's a weak acid...?

Would that reasoning work?

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[laczlacylaci]

Maybe I'm being foolish but don't they tell you the pH? Since [H+]=1E-5, pH is 5, so you instantly know it's acidic. Since the pH is greater than, say, 2 you know it's a weak acid...?

Would that reasoning work?

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Lol I never thought of that. I think that would work.