pKa or pKb?

DrKendrickLamar · Aug 15, 2007

On a practice question, I was asked to calculate the pKa of HCO3-. I was given the Ka=4.7x10^-11 and the Kb=2.7*10^-8 for HCO3-. Why would you use the pKb indirectly to find the pKa?

the choices were:
A. 10-->i got this by pluggin the Ka into the pKa equation
B. 8
*C. 6-->this was the answer by using the Kb
D. 4

wcbcruzer · Aug 15, 2007

FiLLiBEANO said:
On a practice question, I was asked to calculate the pKa of HCO3-. I was given the Ka=4.7x10^-11 and the Kb=2.7*10^-8 for HCO3-. Why would you use the pKb indirectly to find the pKa?

the choices were:
A. 10-->i got this by pluggin the Ka into the pKa equation
B. 8
*C. 6-->this was the answer by using the Kb
D. 4

pKa + pKb = 14

DrKendrickLamar · Aug 15, 2007

dude, i know that. thank you though.

what i am saying is why did the TPR use Kb instead of using Ka when it is given to calculate pKa?

The question is:

What is the approximate pKa for carbonic acid?

axp107 · Aug 15, 2007

Carbonic acid is H2CO3...

ka/kb work for conjugates only...

for ex:

If

pka of NH3 = 10
pkb of NH2- = 4

To find the pkb of NH3, you'd need the pka of NH4+

its tricky.. but once you get it down, you won't make mistakes

wcbcruzer · Aug 15, 2007

The Ka given is for HCO3, not H2CO3. The Kb they gave you is for HCO3 and can be used to get the Ka of H2CO3 (conjugates).

That's tricky. I always get nailed on those, especially when in a hurry. F THE MCAT!

vmp200 · Aug 16, 2007

HCO3(-) + H2O -> H2CO3 + OH- = forward rxn use kb2/reverse of this rxn use ka1
HCO3(-) + H2O -> CO3(2-) + H3O+ = forward rxn use ka2/reverse of thix rxn use kb1

kw = ka1*kb2
kw = ka2*kb1

pKa or pKb?

DrKendrickLamar

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DrKendrickLamar

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axp107

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