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Help with Solubility, please

Discussion in 'MCAT Study Question Q&A' started by geeyouknit, Aug 1, 2011.

  1. geeyouknit

    2+ Year Member

    Jun 22, 2011
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    So I had a question about AAMC 4 #108. I was reading the explanation, and I just wanted to see if someone had a better explanation for why (or if they could just explain solubility).

    "The passage states that compound C (ed note: irrelevant) is soluble in both dilute acid and base. Carboxylic acids are soluble in dilute base (5% NaOH, ex) and amines are soluble in dilute acid. Amides and nitro compounds are not soluble in dilute base or acid."

    Why are acids soluble in base and amines soluble in acids? I feel like this is a stupid question, but help me out please.
  2. plzNOCarribbean

    May 21, 2010
    Likes Received:
    are you talking about a salt? A common rule of thumb is that:
    (1) basic salts are more soluble in acidic solutions and
    (2) Acidic salts are more soluble in basic solutions

    The reason is because of a complex equilibrium. For example, the salt sodium acetate is a basic salt, which dissociates to form Na (aq) + CH3COO- (aq) in solution. Virtually all of the Na+ will be dissolved in solution (its a group I metal) and the base component, CH3COO will react with water to form OH ions. The complex equilibrium looks like this:

    Na CH3COO (s) ==> Na (aq) + CH3COO (aq)

    CH3COO (aq) + H2O ==> CH3COOH + OH

    Now, if this complex equilibrium were to occur in the presence of acid, the acid would eat up the OH. This is like a stress being applied to a system, as if we were removing product, which would cause our equilibrium to shift Right. As a result, more of that salt would have to dissolve in order to compensate for the reduction of [OH] in the presence of acid. This is why bases are more soluble in acids and acids are more soluble in bases. This generally holds true.
  3. collegestud2013

    collegestud2013 Probationary Status
    7+ Year Member

    Jun 28, 2010
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    Resident [Any Field]
    A base will deprotonate a neutrally charged acid to give a negatively charged conjugate base, which has an overall -1 net charge and is thus more polar. Since water is also polar and like dissolves like, the increased similarity in IMFs allows for easier solvation.

    Same argument for an amine. A strong acid will protonate it, giving an overall +1 charge and also making it more polar.

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