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taken from TPR test D science

Discussion in 'MCAT Discussions' started by jhk43, Mar 20, 2004.

  1. jhk43

    jhk43 Senior Member
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    i found this exam pretty hard.

    Q1. Why is CO3(-2) strongly basic in water? ph=11+. shouldnt it be mildly basic, since its the conjugate base of a very weak acid?

    Q2. Lets say you neutralize 99.9% of a pH 1 acid solution. what pH are u left with? 0,3,6,7?
     
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  3. premyo2002

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    I haven't reviewed Acid/Base chemistry, but I thought that if a an acid is weak, this means it wants to hold on to protons (H), So the conjugate base, would want to hold on to protons. This means weak acid--->stronger base, weak base----> stronger acid


    As for your second question, it should be about a little less than 7 if you neutralize most of it with a weak base, if you use a string base it would be 7. They are basically asking, "what's the equivalence point"
     
  4. rugbyboy

    rugbyboy Member
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    1) Conjugate base of a weak acid is a very strong base. Also true vice versa HCl very strong acid , Cl- very weak base.

    2) 3
     
  5. jhk43

    jhk43 Senior Member
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    Q1. Why is CO3(-2) strongly basic in water? ph=11+. shouldnt it be mildly basic, since its the conjugate base of a very weak acid?

    HCO3- <--> H+ CO3(-2)

    HCO3- is a mildly weak acid. CO3(-2) should be a mildly weak base.

    HCl is a strong acid. conjugate base is a VERY weak base (negligible).
     
  6. Munchkin6245

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    Wouldn't question #2 be ph = 7 since it says "neutralize" and a neutral pH IS 7???? Just wondering...not answering
     
  7. ASDIC

    ASDIC The 9th Flotilla
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    pH 1 means... [H+] = 1 x 10 ^-1

    so 0.1 % of [H+] remains which equals ((0.1/100) x 1 x 10^-1)

    = 1 x 10^-4 ==> pH ~ 4

    4 is close to 3.

    on the mcat...they will not give exact answer in the choices. So, Kaplan advises that one should just make an approximation.
     
  8. Cerberus

    Cerberus Heroic Necromancer
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    CO3(-2) <-> HCO3- +H3O+<->H2CO3 +H2O <-> H2O+CO2

    :)
     

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