weak bases/acids

[junkct]

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Um so this might be a stupid question, but if something is a weak acid, does that ALWAYS mean that it's conjugate base is a strong base (and vice-versa)?

Also, at what Ka/pKa or Kb/pKb does the MCAT consider an acid/base as a WEAK acid/base?

 

[engineeredout]

I think a pka > 1 is a weak acid, and pkb > 1 is a weak base. They won't try to trick you and say "If an acid has a pka of 1, is it a strong acid, weak acid, strong base... etc", you'll more likely see extremes like a pka of 55 or 10^-20

 

[junkct]

Yeah I know, that's what I thought too. I was just doing a Kaplan FL though and they gave Ka and Kb values for HCO3- and asked whether it was strong acd/weak base or weak acid/weak base or strong base/weak acid, etc. So I was just wondering if it was important to know.

But your pKa/pKb > 1 makes a lot of sense, because if it is >1 then that means more reactants than products

 

[BerkReviewTeach]

Yeah I know, that's what I thought too. I was just doing a Kaplan FL though and they gave Ka and Kb values for HCO3- and asked whether it was strong acd/weak base or weak acid/weak base or strong base/weak acid, etc. So I was just wondering if it was important to know.

But your pKa/pKb > 1 makes a lot of sense, because if it is >1 then that means more reactants than products

Be careful not to cross up logs and regular numbers.

If pKa = 1, then Ka = 10exp-1 = 1/10 That means that there is 1 product for every 10 reactants, with favors reactants.

I think the previous poster meant to say Ka > 1 equates to a strong acid, which correlates to pKa < 0.

Also, to answer your weak acid-strong base conjugate pair question, let me ask you a question.

• Ammonia is a weak base and has a pKb of 4.7. How is ammonium, its conjugate base, best described?
  A) Strong acid
  B) Weak acid
  C) Strong Base
  D) Weak Base

The answer to that question will answer your question.

 

[junkct]

Be careful not to cross up logs and regular numbers.

If pKa = 1, then Ka = 10exp-1 = 1/10 That means that there is 1 product for every 10 reactants, with favors reactants.

I think the previous poster meant to say Ka > 1 equates to a strong acid, which correlates to pKa < 0.

Also, to answer your weak acid-strong base conjugate pair question, let me ask you a question.

• Ammonia is a weak base and has a pKb of 4.7. How is ammonium, its conjugate base, best described?
  A) Strong acid
  B) Weak acid
  C) Strong Base
  D) Weak Base

The answer to that question will answer your question.

Well that would mean the pKa of ammonium is 9.3, which would make it a weak acid...

 

[BerkReviewTeach]

Well that would mean the pKa of ammonium is 9.3, which would make it a weak acid...

Exactly, which means that for conjugate pairs, they can be strong-weak or weak-weak.

Ammonia is a weak base and its conjugate acid is a weak acid. The reason it sounds odd, even though the numbers show it to be true, is because whenever they talk about ammonia/ammonium, it's usually in reference to a buffer. A buffer is a mixture of a weak acid and its weak conjugate base in roughly equal portions.

If you have a friend who has the BR general chemistry books (book I in particular), it is more than worth reading the acid-base and the titration chapters, because they thoroughly explain acid-base chemistry in simple, conceptual terms. I've learned more teaching from that book than I ever learned in school about how to think about acids and bases questions.

 

[junkct]

Wow, thanks a lot for the info. Makes a lot more sense now! Yeah, I pretty much understand the acid/base stuff in my study material, but I still seem to have a hard time with acid/base questions on my practice tests. Maybe I do need another way of looking at it. I'll def check out the BR books. Do you know if they are available in stores at all? Or is the only way to get them through the course?

 

[BerkReviewTeach]

Wow, thanks a lot for the info. Makes a lot more sense now! Yeah, I pretty much understand the acid/base stuff in my study material, but I still seem to have a hard time with acid/base questions on my practice tests. Maybe I do need another way of looking at it. I'll def check out the BR books. Do you know if they are available in stores at all? Or is the only way to get them through the course?

They are on line through BR or via eBay used. But, if you are taking the MCAT this weekend, the only realistic way to read that in time would be to borrow a friend's copy. Maybe someone at SDN can lend it to you.