pKb for weak/strong base?

[MedPR]

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So I didn't think there was a cutoff, or that any question like "HA pKa = 4.5, is A- a weak base or a strong base?" would show up. I just did one in EK 1001 and I feel that a pKb of 9.5 is good enough to be considered a strong base. EK says pKb=9.5 is a weak base.

At what point do we consider it a strong base?

 

[pm1]

So I didn't think there was a cutoff, or that any question like "HA pKa = 4.5, is A- a weak base or a strong base?" would show up. I just did one in EK 1001 and I feel that a pKb of 9.5 is good enough to be considered a strong base. EK says pKb=9.5 is a weak base.

At what point do we consider it a strong base?

I thought that the stronger the base, the bigger the Kb and the smaller the pKb. I would say that a pKb of 9,5 is pretty high, hence a weak base.

and that is because Kb = [B+][OH-] / [BOH. The stronger the base, the more it will dissociate, giving a bigger Kb, or smaller pKb.

 

[folktale]

I also get tricked by these types of problems before. But yes, just remember that high Ka and low pKa are strong acids, which is also similar with bases. High Kb and low pKb means strong bases.

 

[MrNeuro]

So I didn't think there was a cutoff, or that any question like "HA pKa = 4.5, is A- a weak base or a strong base?" would show up. I just did one in EK 1001 and I feel that a pKb of 9.5 is good enough to be considered a strong base. EK says pKb=9.5 is a weak base.

At what point do we consider it a strong base?

pKa/pKb < 0 = Strong
pKa/pKb 0-14 = Weak
pKa/pKb > 14 = Very Weak

 

[MedPR]

Oh crap that's right. I'm dumb. Thanks.

 

[gettheleadout]

The way I look at it is if the pKb is 9.5 then the Kb = 10^(-9.5) and that is a tiny number, meaning a very small amount of HB formed from B-, so the base mostly remains base instead of reacting, therefore it is a weak base.

 

[30somethingyearold]

I realise that this post was written years ago but just in case anyone is looking at this and revising I thought I would post something.

I found this quizlet link which has a good summary of pka and pkb, etc: http://quizlet.com/11133451/general-chemistry-for-mcat-flash-cards/

Basically, large Ka (small pKa) = at equilibrium there are more products more reactants, meaning a very strong acid (reaction lies to the right). Same is true for Kb, a large Kb (small pKb) = strong base.

 

[AGSIII]

The way I was taught kb> 1 is a strong base. Remember strong bases fully disassociate in water and drive reactions to completion