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Offline 1stplace

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weird pH indicator question
« on: April 05, 2008, 09:33:10 PM »
Question: A solid sample containing only sodium hydrogen oxalate (NaHC2O4) and oxalic acid (H2C2O4) is dissolved in 100.00 mL water. In a first experiment, 10.00 mL of this solution is reacted with 10.00 mL of a 0.100 M solution of sodium hydroxide (NaOH). It takes 7.00mL of a 0.100 M solution of hydrochloric acid (HCl) to titrate the resulting mixture up to the end point indicated by the color change of the thymolphthalein indicator.

pKa1=1.25
pKa2=4.27
Knowing that the tymolphathalein indicator changes color in a pH range of 9.4 to 10.6, the concentration of which basic species then present in solution can be determined by this titration?
Circle all valid applicable and justify your answer briefly
OH- C2O4(2-) HC2O4(-) H2O Cl-
-------

I really don't know how this titration will determine any of these compounds. First of all, if all of it were in C2O4(2-), the solution would still be acidic, due to the low pKa2. So, the only way that the indicator can change color is for all of the compounds to be in C2O4(2-) with excess base. Nevertheless, it is still very confusing. Can someone explain this titration to me?

Offline Arkcon

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Re: weird pH indicator question
« Reply #1 on: April 05, 2008, 09:40:36 PM »
You are given pKa's for the starting chemicals, do you know what those mean?

You're given a pH for the color change, and you're told the color change happens.  So you know what the pH is, or at least what it's not, right?

Can you draw an x through some ions that you know can't be there, as a consequence?  That'd be a start.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline 1stplace

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Re: weird pH indicator question
« Reply #2 on: April 05, 2008, 10:31:16 PM »
The pKa are -log(Ka) and show the acidic dissociation.....what i was trying to say is in my post is that these pKas (both pKa1 and pKa2) are pretty low, which means that even at the acid's second dissociation, it is probably going to be acidic. So, the only way that the pH range can even touch the pH range of the indicator is when there is excess base.

Anyways, the question does not ask what ions will be there....it asks which ion's concentration can be determined thorough this titration.

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