AAMC Q's From Official Guide, Physical Sci #24

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avo
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Joined: Wed Jun 12, 2019 6:01 pm

AAMC Q's From Official Guide, Physical Sci #24

Post by avo » Thu Jul 25, 2019 12:31 am

Hi! It might be that I'm not that great in understanding how titrations work, but why was it that we had to dissolve the precipitate in acid, and not base? (answer choices B/D have the same solutions but switched).
NS_Tutor_Mathias
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Re: AAMC Q's From Official Guide, Physical Sci #24

Post by NS_Tutor_Mathias » Thu Jul 25, 2019 6:20 pm

The easiest way to imagine why this is the precipitate of a base is to imagine where that chain attached to the M2+ cation came from: From a deprotonated fatty acid. That should give you a weak base - or a nucleophile or electron donor if you will. You definitely know from that point on that an acid and a base would form water and a salt - so adding acid to a basic salt should most definitely dissolve it (now forming M2+ + 2 Cl- plus fatty acids in solution)

You can also imagine what would happen in the presence of a base, like NaOH: Nothing, because you already know that if both Na+ and M2+ are in solution, the soapy precipitate forms (reaction 1). The only way to undo this is to start protonating that stearate.

As a fun note: There is a significant mistake here, stearate (the conjugate base of stearic acid) has 35 protons, not 32.
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