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Titration of Weak Acid EK 1001

Discussion in 'MCAT Study Question Q&A' started by MedPR, Mar 27, 2012.

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  1. MedPR

    MedPR

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    EK Chem number 848.

    There's a figure showing the titration curve of 0.1M carbonic acid with 0.1M NaOH. You don't really need the figure to answer my question, but if it helps you, this is page 91 of EK 1001 Gen Chem.

    If the concentration of carbonic acid were greater, how would the titration curve change?

    A. pH of the starting point would be lower and the pH of both equivalence points would be greater
    B. pH of the starting point and both equivalence points would be lower
    C. pH of both half equivalence points would be greater
    D. pH of both half equivalence points would be lower

    Answer is: A

    I understand the first part of the answer (pH starting lower), but not the second part. Why wouldn't the equivalence points be at a lower pH too? EK's explanation is
    I mean, I guess the explanation helps a little (I understand why more base = higher pH). Based on the explanation I understand how the second equivalence point (all CO32-) would be higher, but the conjugate of carbonic acid is still an acid (HCO3-), just not as strong of an acid as carbonic acid itself. So shouldn't its basic/acidic properties kind of cancel each other out and be at the same equivalence point as before?
  2. chiddler

    chiddler

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    Acidity (pKa) 10.3
    Basicity (pKb) 3.7

    it seems it would be more basic than acidic. so it should absorb protons after the initial neutralization.
  3. SaintJude

    SaintJude

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    Wait...I thought the pH depended on it's molarity? Why would pH starting point be lower?
  4. pfaction

    pfaction

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    Molarity previously was 0.1 moles in one liter.

    Increasing concentration means 0.1moles+x which is >0.1 moles, thus immediately increases molarity, which means more H+ dissolved, so pH decreases.

    As for the rest of the questions, idk. I know as you increase the molarity, more will need to be converted to a base, requiring more base. The conj base is after all a weak base, so I thought it would get shifted upwards as well.
  5. MrNeuro

    MrNeuro Gold Donor

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    if you increase the concentration of carbonic acid that means at equivalence pts 1 and 2 the pOH = 1/2pkb - 1/2log[A-] if you increase the concentration the pOH decreases meaning pH increases...

    i worked out by saying that if you increase the concentration that means at equivalence the mols of conj acid = mols of base added if you increase the mols of conj acid you must add more mL of base so the equivalence should be shifted higher because we know the buffer range isn't affected and the beginning just has a greater slope....hence the equiv must be higher as its adjusting

    writing this has me wondering if the titration curve shows a smaller slope at the equivalence pt part...hmmmmm i think it might
  6. chiddler

    chiddler

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    I don't like this reasoning because it does not delve the details.

    rather

    the first equivalent point is when all the acid has been neutralized by one part base.

    so i think we can rephrase the question as follows: what effect does adding more HCO3(-) on an HCO3(-) solution?

    Looking at the data I posted above, I think that it is more basic than acidic so it would make sense that it would absorb protons rather than give. If we use something like oxalic acid, pH1 and 2 are like 1 and 4, I think the pH would be lower.
  7. gettheleadout

    gettheleadout title_error Moderator

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    Can someone please tell me where there are multiple equivalence points on a titration curve? There are two half-equivalence points but isn't there only one equivalence point?
  8. MrNeuro

    MrNeuro Gold Donor

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    there are two for this weak diprotic acid

    http://www.sjsu.edu/faculty/chem55/55acid0.jpg
  9. MrNeuro

    MrNeuro Gold Donor

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    HUHHH? :confused:
  10. pfaction

    pfaction

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    I don't want to derail the topic, but I'd like to ask a Q about this.

    Looking at that picture: [​IMG]

    Let's say we flipped it, we're looking at a weak base, so the bottom axis is Vol HCL. Then when it says pH=pKa, it should be pH = pKb, to find the pKa do 14-pKb?
  11. chiddler

    chiddler

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    can you explain what you're confused about, please.

    as i wrote earlier:

    i think we can rephrase the question as follows: what effect does adding more HCO3(-) on an HCO3(-) solution?
  12. MrNeuro

    MrNeuro Gold Donor

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    that only applies at 25℃ otherwise i dunno
  13. gettheleadout

    gettheleadout title_error Moderator

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  14. pfaction

    pfaction

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    Remember the lungs for H2CO3. In our body, CO2+H2O->H2CO3, which makes H+ and HCO3-. In the lungs, the process is reversed! Conservation of atoms and all that good stuff.
  15. Temperature101

    Temperature101

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    I did not get it. I thought the answer should be B..Can someone explain why its not B.
  16. gettheleadout

    gettheleadout title_error Moderator

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    At the (or in this case, "an") equivalence point an amount of strong base equal to the amount of weak acid has been added to the solution. This means all of the weak acid will deprotonate, being converted completely to conjugate base. The only remaining major species in the solution is this conjugate base, so the pH of the solution is based off the equilibrium it forms in solution (essentially A- + H+ <-> HA). Because the conjugate base is stronger than the weak acid it originated from, the solution will thus be slightly basic, to a degree proportional to the amount of conjugate base present. If you increase the initial concentration of weak acid and then take the titration to equivalence, you now have more conjugate base present than you did with a lower amount of initial weak acid. More conjugate base present at equivalence means more basic solution from the resulting equilibrium. More basic solution means higher pH, not lower.
  17. Temperature101

    Temperature101

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    Got it...thanks
  18. pfaction

    pfaction

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    Ah, so we can think of this as a sort of actual widening gap. Since the molarity increases, it becomes more acidic, but the tails to that coin is, since there's more weak acid, there will also be more conjugate base driving up the pH, so at equivalency point, it'll be even more basic! Good logic GTLO.
  19. chiddler

    chiddler

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    remember that this depends on the type of acid, though.

    like i mentioned earlier, for something like oxalic acid with a low pKa 1 and 2, the results would be different.
  20. pfaction

    pfaction

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    Because it's a strong acid, right? It would have around 7?

    Oh wait, it's diprotic, damn. Now I don't know.
  21. chiddler

    chiddler

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    because both of the protons on oxalic acid are acidic.

    you can't say the same thing about carbonic acid. after releasing its first proton, it becomes a better base than an acid.
  22. gettheleadout

    gettheleadout title_error Moderator

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    A very good point. In the case of oxalic acid (if you increased initial concentration) the first equivalence point would actually have a lower pH, while only the second would have a higher pH.

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