[Chaste]

Calculate the pH of the resulting mixture when 20.0 mL NaOH 0.01 M is added to 20.0 mL of oxalic
acid 0.01 M.(Ans = 3.325)

pk1 of oxalic = 1.25
pk2 of oxalic = 4.266

anyone has time to look at my question?
This is my attempt at the question...I can't seem to be able to find [A^2-]. Somewhere in the middle, I wrote wrongly, it should be [H2A] << [HA-] is valid.

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

[Chaste]

Hi Borek, yes, I've followed that method. However, the assumption used in the website which is CNaA- = [HA-] is invalid for my case. Thus, I'm unable to obtain the right answer. I need to find the new [HA-]

which is CNaA- - [A2-]. But I can't seem to obtain [A2-]

[Borek]

However, the assumption used in the website which is CNaA- = [HA-] is invalid for my case.

Why?

[Chaste]

However, the assumption used in the website which is CNaA- = [HA-] is invalid for my case.

Why?

As you can see from my workings, [A2-] will affect the results significantly. Because it's not negligible as the website assumed.

CNaA- = [HA-] + [A2-] + [H2A] in actuality, and this is so for my case where the [A2-] cannot be ignored.

[Borek]

OK, I see. You are right, oxalic is a too strong for some of the approximations.

Check if ignoring pKa1 (that is, assuming no hydrolysis, only presence of acid HA-) won't give reasonable result. Once you calculate pH this way you can easily estimate concentration of H₂A - if it is low, assumption was good enough.

[AWK]

As the first approximation use equation 12.7 from Borek primer. It gives a result quite close to the expected one.

[Chaste]

As the first approximation use equation 12.7 from Borek primer. It gives a result quite close to the expected one.

yes with equation 12.7 that I used, I would get pH of 3.3. However, the answer is much more precise 3.325 because the concentration [A2-] cannot be ignored. My interested quantity is still finding [A2-] and hence the exact answer 3.325.

[AWK]

Now you can start an iteration procedure.