pKa or pKb?

wcbcruzer

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• Aug 15, 2007

• #2

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

Click to expand...

pKa + pKb = 14 👍

 

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DrKendrickLamar

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• Aug 15, 2007

• #3

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?

 

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axp107

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• Aug 15, 2007

• #4

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

 

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wcbcruzer

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• Aug 15, 2007

• #5

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!

 

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V

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• Aug 16, 2007

• #6

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

 

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