[daylight]

I have a problem that I need help with understanding why it is set up this way.
Calculate pH of 0.100M solution of Na2C2O4. for H2C2O4 K_a1=5.9x10^-2; K_a2= 6.4x10^-5.
the solutoins has:
Cr2O4^2-, K_b1+Kw/K_a2, and K_b2=Kw/K_a1.
so why is b1 not kw/ka1 and b2 not kw/ka2? this is confusing if Kb=Kw/Ka *delete me*??

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=polyprotic-dissociation-constants

Scroll down the page.

Although I am not sure what you are really asking about, as there are several typos in your post.

[daylight]

OK< I don't know how the 'delete me' got in there, something I clicked I guess, but I am asking why is Kb1= to Kw/Ka2 and not Kb1=Kw/Ka1...

[enahs]

Write the reactions of Ka1 and Ka2.
Write the reactions of Kb1 and Kb2.

Which one are compliments of each other?

[Borek]

but I am asking why is Kb1= to Kw/Ka2 and not Kb1=Kw/Ka1...

Have you visited the page at link posted? Have you read it?

[daylight]

H2C2O4 + H20   H3O + HC2O4
HC2O4 + H20   H30 + C2O4

C2O4 + H2O   HC2O4- + OH-
I'm not sure if I should then have H2CO4 or what?

[daylight]

Yes, I've read it, but it's still confusing. It would help if it were a real problem...

[Borek]

OK, your oxalic acid is almost OK, just please DON'T ignore charges. You may wrote H⁺ between products, while we all know it is H₃O⁺ you will have to ignore water in dissociation constants, that's additional step tham may lead to confusion.

You need second hydrolysis reaction, and you are right - undissociated oxalic acid will be between products.

Now, write definitions of Ka and Kb for each reaction. Assign numbers (a1, a2, b1, b2) to them.

[daylight]

ka1= H2C2O4 + H20   H3O+ + HC2O4
ka2= HC2O4 + H20   H30+ + C2O4-

Kb1= C2O4- + H2O   HC2O4 + OH-
Kb2= HC2O4- + H2O  H2C2O4
I'm not sure about b1 b2, I haven't done one like that before... so then Kb2 would be the reverse of ka1 and kb1 the reverse of ka2?

[Borek]

You are still missing some charges, and I would not wrote

ka1= H2C2O4 + H20   H3O+ + HC2O4

as

K_a1 = [H⁺][HC₂O₄^-]/[H₂C₂O₄]

but you are gettng it - Kax and Kbx are numbered in the opposite directions, so Kafirst*Kblast=Kw and so on.