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Did anyone have a question about hydroflouric acid on the MCAT?

Discussion in 'Pre-Medical - MD' started by gabbers22, Aug 21, 2001.

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  1. gabbers22

    gabbers22 Member
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    Okay...I know that I shouldn't torture myself by asking about questions that I think I might have missed...but I thought I would do it anyway.
    There was a passage on my PS section reagarding HF acid and when it burns, Ca is depleted from the body. Anyway, my uestion is really stupid...there was a Le Chatlier question regarding a reaction like this:
    HF reacting to form H+ ions and F- ions. The question asked if OH was added which direction would the reaction be pushed and which species would it react with.
    The choices were
    a. react with H+ and push to left
    b. react with H+ and push to the right
    c. react with HF and push to left
    d. react with HF and push to right

    Now, I thought since it did not specifically say which side it would add to...that the people probably added it at the beginning of the reaction, therefore it would react with HF and push to the right (d).
    What do you all think? Not that I can change it now...but I thought I might as well ask.
    Oh...and here's another what is the better base? F, I, Cl or Br?
    Thanks--
    gabbers22
     
    #1 gabbers22, Aug 21, 2001
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  2. Sm00th13

    Sm00th13 Senior Member
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    According to the question, it's deals w/ LeChatlier principle. If you add -OH to the solution, it would react w/ H+. Since there is less of H+ ion, the reaction will go to the right. I would say in my judgement, the answer is "B". When you say a "better" base, do you mean a stronger base (which would be "F")?
     
    #2 Sm00th13, Aug 21, 2001
  3. serpiente

    serpiente Senior Member
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    As a chem major, I'd have to say B is the answer. It's Le Chatlier's plus HF is a weak acid, so you basically have a buffer situation. The HF is in eqilibirum with the H+ and F-. When you add base, the base reacts with the Ht. The rxn goes to the right to replace the lost H+. About your other question, Note HI is the stongest acid, I- the weakest base (periodic trends). Thus F- is the strongest base.
     
    #3 serpiente, Aug 21, 2001
  4. moo

    moo 1K Member
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    I agree with the others, B is correct. Your question of "what side" it's added to is flawed. There is no "side." If you put some HF into water, we'll get H3O+ and F-, along with some HF still left in the water UNDISSOCIATED. The OH- will react with the H3O+ (i.e., H+). Since the [H3O+] decreases, the eqbm will shift right to compensate.
     
    #4 moo, Aug 21, 2001
  5. gabbers22

    gabbers22 Member
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    Yeah--after seeing all you guys reasoning, I see that you are right...the funny thing is that I originally answered the question the way you said, however, I double guessed myself and changed it at the last minute. Darn...what was I thinking!
    Thanks
    -gabbers22
     
    #5 gabbers22, Aug 21, 2001
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