Calculating pKa from pKb (not what you think)

[MedPR]

What is the pKa for ammonia, given that the pKb for ammonia is 4.7

A. 4.7
B. 7.0
C. 9.3
D. 33

If you don't fall for the trap, then the answer is obviously 33. NH3 is a weaker acid than NH4+, and the pKa for NH4+ is 9.3 (given the pKb for its conjugate base, NH3, is 4.7), therefore, the pKA for NH3 must be greater than 9.3. Since there is only 1 answer choice greater than 9.3, that's the answer - D.

However, what if there was more than 1 answer greater than 9.3. Is there a mathematical, or conceptual way to estimate the actual pKa for something given its pKb?

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[MT Headed]

No. You have no information on how 'sticky' that next proton is on ammonia, other than it obviously must be stickier than the proton of NH4+.

This is a good question to catch the unaware 🙂

 

[MedPR]

No. You have no information on how 'sticky' that next proton is on ammonia, other than it obviously must be stickier than the proton of NH4+.

This is a good question to catch the unaware 🙂

Yea, especially on a test that rewards ruling out "obviously wrong" answers right away.

 

[Pisiform]

so let me get this straight ... pKa + pkb = 14 for only conjugates in this case Ammonium and ammonia ??

 

[MrNeuro]

so let me get this straight ... pKa + pkb = 14 for only conjugates in this case Ammonium and ammonia ??

yeah

 

[Pisiform]

yeah

cool thanks

 

[ThirdEye]

Great example. I've fallen for this trap too often

 

[MedPR]

so let me get this straight ... pKa + pkb = 14 for only conjugates in this case Ammonium and ammonia ??

Yea it's something so simple that you forget all about the part where it's only for conjugates.

Great example. I've fallen for this trap too often

I would've fallen for it too had I not read the section in TBR leading up to the question.

It's a pretty good trap imo.

 

[Ssina]

Great example. I've fallen for this trap too often

👍
thanks for sharing MedPR