[JohnYGA]

I have conducted a lab in which H2C2O4 is titrated with NaOH (0.112 mol dm-3). How would I go about determining the acid dissociation constant Ka for oxalic acid?

[FreeRadical]

Do you have a titration curve (pH vs volume of NaOH)?

[JohnYGA]

Do you have a titration curve (pH vs volume of NaOH)?

No, the procedure we were given did not involve measuring pH

[rabolisk]

OK, so how did you titrate? That is, how did you know what the end point was?

[JohnYGA]

The acid was added to the NaOH with phenolphthalein until the solution changed from pink to colorless

[Borek]

No pH, no curve, no Ka.

[FreeRadical]

No pH, no curve, no Ka.

That's right. You simply don't have enough information to find a Ka.

[JohnYGA]

Okay thanks.  So just to be clear, without pH there is no possible way to determine Ka from titration?

[rabolisk]

Not precisely. I suppose you can take the end point to be the equivalence point, and then use that to figure out pKa2, but given the wide range of end points for phenolphthalein, this is not precise by any means.

[Borek]

Not precisely. I suppose you can take the end point to be the equivalence point, and then use that to figure out pKa2, but given the wide range of end points for phenolphthalein, this is not precise by any means.

No, that won't get you far. Phenolphthalein can be used for almost any acid strong enough, titrated with strong base. Strong enough means in practice anything with pKa below 7 or 6.