there you go.
Now, you know the concentration of the sodium hydroxide you said in your first post, right?
Or you determined it, either way I am going to assume you know it)
So you know the concentration of OH-
in your solution, right?
then check this: http://en.wikipedia.org/wiki/Phenolphthalein
for you the transition that matters is the pink-to-colorless transition at higher pH, which occurs at a pH of about 8.2.
So now you know the original concentration of OH-
, you know the amount of oxalic acid you added, and you know the end concentration of OH-
so you can calculate how many H+
of the oxalic acid was used to neutralize the NaOH.
which should give you the amount of C2
you have left in solution. And from the pH you know the [H+