polyprotic acids and pKas + pKbs

[Graffiti]

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for this reaction:

h2co3(aq) + h2o(l) ---> h3o+(aq) + hco3-(aq)
hco3-(aq) + h2o(l) ---> h3o+(aq) + co3 2-(aq)

if we're told the pka1 for a polyprotic acid like h2co3, why is it that the pkb1 (found by 14-pka) corresponds to co3 2- (NOT hco3-)? i guess im confused because i thought that the pkb always corresponded to the conjugate base of the acid. maybe im misunderstanding something here??

and is this something we're suppose to know for the mcat because it's really confusing.

 

[Graffiti]

if anyone could help, that would be great. or atleast tell me if that's right.

 

[GreenRabbit]

H2CO3 <---> HCO3^- <---> CO3^2-

Hmm, if you assign pKa1 and pKb2 to the first reversible reaction, and pKa2 and pKb1 to the second reversible reaction (as per convention), I believe..

Kw = Ka1*Kb2, 14 = pKa1 + pKa2
Kw = Ka2*Kb1, 14 = pKa2 + pKa1

I think you may be confused with the naming convention. If pKb1 corresponds to CO3^2-, then pKb1 does not equal 14 - pKa1.

 

[Graffiti]

H2CO3 <---> HCO3^- <---> CO3^2-

Hmm, if you assign pKa1 and pKb2 to the first reversible reaction, and pKa2 and pKb1 to the second reversible reaction (as per convention), I believe..

Kw = Ka1*Kb2, 14 = pKa1 + pKa2
Kw = Ka2*Kb1, 14 = pKa2 + pKa1

I think you may be confused with the naming convention. If pKb1 corresponds to CO3^2-, then pKb1 does not equal 14 - pKa1.

ah, I see. so basically it's the same idea, just weird naming.
just to make sure, could you tell me if this is right?

14-pka1 (h2co3) = pkb2 (hco3-)
14-pka2 (hco3-) = pkb1 (co32-)

 

[GreenRabbit]

ah, I see. so basically it's the same idea, just weird naming.
just to make sure, could you tell me if this is right?

14-pka1 (h2co3) = pkb2 (hco3-)
14-pka2 (hco3-) = pkb1 (co32-)

That looks right.