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Acid indicator question

  1. Apr 17, 2005 #1
    I don't undestand this question. How can i determine which will be the major component?

    HCl + Na2CO3 - > NaCl + H2O + CO2

    Is it water?

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  2. jcsd
  3. Apr 17, 2005 #2


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    You see dominant colors when one color is 10x as concentrated as another, so the general rule is that you see an indicator end point ~1 pH from the pKin value. That means the endpoint is ~4.7 pH (still acidic), and there is still HCl left in solution (this is important)

    I would say it's either water or NaCl. This whole thing is aqueous, so water seems like the obvious answer.
    For every HCl reacting with any Na2CO3 or NaHCO3, you get NaCl.
    Carbonic acid, bicarbonate, and carbonate are all in a weird equilibrium, so none of them can claim 100% the way NaCl can.

    The answer the teacher wants is probably NaCl.
  4. Apr 17, 2005 #3


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    Remember at the first equivalence point, the solution is basic.

    I'm not quite sure about this one at the moment...D seems appealing, since it is the salt that is produced in the reaction between the acid and base.
  5. Apr 17, 2005 #4


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    But this titration never makes it to the equivalence point.
  6. Apr 17, 2005 #5


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    I just hope I haven't mix the constants...

    [H+] = 1.322e-9
    [Cl-] = 0.0671
    [CO3(2-)] = 0.03322
    [HCO3(-)] = 7.149e-5
    [H2CO3] = 2.028e-11

    Water is the only answer that holds. I hate questions where you have to guess what teacher wants to know :(

    First equivalence point has pH = 3.92, so not far. pH = 3.7 for the 99.81% titration.

    Chemical calculators for labs and education
    BATE - pH calculations, titration curves
  7. Apr 17, 2005 #6


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    I think it's a trick question. Look at the wording of it
    "which of the following species is the major component of the solution at the methyl orange endpoint"
    Notice how it doesn't say product? If it said product, the answer would be NaCl. Since it says component, just say the water. If you answer NaCl and the teacher wants water, you don't have a leg to stand on. If you answer water and he marks it wrong, the teacher doesn't have a leg to stand on.
  8. Apr 17, 2005 #7
    ROFL, I read the question again and I think you hit the nail on the head ;)
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