[UnderneathTheLens]

So I have been doing this problem for the past few days, and no matter what I try, I can't get the right answer (60mL)

The Question is: What volume of 0.120 NaOH must be added to 100mL of 0.100 M NaHC2O4 to reach a pH of 4.7?

The Ka of H2C2O4 is 5.9*10^-2, the Ka of HC2O4- is 5.1*10^-5

So in my most recent try, I had HC2O4 + H2O <-> H3O+ + C2O4-

With initial values of A for HC2O4 and 0 for the rest. I then ICEd it, and had x = to 1.995x10^-5 (-log(1.995x10^-5) = 4.7, so I'm simply working backwards here)

Then I solve for A, using Ka of HC2O4 and then solve for the volume needed. However, this doesn't yield the right answer. It gives me an answer in the hundreds of thousands. I'm completely out of ideas here, does anyone have insight to how to do this?

Thanks in advance.

[Borek]

So in my most recent try, I had HC2O4 + H2O <-> H3O+ + C2O4-

Where is the added OH^- taken into account?

Hint: simplest approach would be to treat the solution as a buffer containing HC₂O₄^- and C₂O₄^2-...