Bootcamp: Conjugate acid of a acid???

[Bigbirdo]

1. If the conjugate base of a molecule has a pKb of 1.4, what would you expect the conjugate acid to be?
   • A. Strong acid
   • B. Weak acid
   • C. Neutral
   • D. Weak base
   • E. Strong base

The correct answer is B. Since pKa + pKb = 14, and pKb is given as 1.4, then pKa must be 12.6. Higher pKa’s correspond to weaker acids, so B is the correct answer. The answer cannot be (E) because the question asked what we expect the conjugate acid to be, and the term “strong base” is incorrect in reference to a conjugate acid. The more appropriate answer would be a weak acid, or answer B.

My questions:
Because the conjugate base has a low value of pkb =1.4, so I would say the original molecule is a weak acid. However, the question asks what you expect the conjugate acid to be. This does not make sense to me. Is it asking the conjugate acid strength of a weak acid?

---

Members don't see this ad.

 

[jwan14]

Conjugate acids and conjugate bases are always referred to in conjunction with each other. If we are referring to the conjugate base of a molecule then that molecule is also known as the conjugate acid. The conjugate base of HCl is Cl-. Thus, HCl is the conjugate acid in this context. I think that might answer your question.

 

[shaidester]

What exactly is the cutoff value between a weak acid and weak base?

 

[dhk2131]

I wouldn't worry about the cutoff of a weak vs strong acid/base. just memorize the known strong acid and bases. you should be familiar with some other weak acids like acetic acid etc. In general, a low pkb means a high kb, which means a strong base. A strong base has a weak conjugate acid (either a weak acid deprotonated to form the strong base, or the strong base protonated to become a weak acid). You do not necessarily have to convert pKb to pKa and back to Ka (but you could of course, since you know that it would have a pKa of 12, which means it has a lower Ka, and thus would be a weak acid.

I would say that a weak acid is one that is anywhere between 5-7 and a weak base 7-9 (if you had to split the 1-14 range into roughly 4 equal parts). If anything, the DAT will either give you the Ka/Kb/pKb/pKa values, or present you with an obvious strong/weak acid/base. They won't give you a pKa value and ask if it is a strong, very strong, or weak acid or not (they'll probably have you compare it to other pKa/Ka/pKb/Kb values). If they give you acetic acid or HI, you should know which is weak and which is strong generally/conceptually (HI is strong because the anion is very stable/weak since the I anion's size can delocalize the charge better).

 

[Don'tSleep]

I'm actually confused about this myself. I just took this test. His explanation doesn't make sense to me either. If the conjugate BASE has a pKa of 12.5 that means the conjugate BASE is a weak acid. Therefore, the conjugate ACID should be a very strong acid right? because a strong acid gives a weak conjugate base. so if that conjugate base is still an acid that means that the original acid is a strong acid.

When i saw this question I immediately thought H2SO4. So I put strong acid (A)
Can someone explain this to me pls?

 

[Don'tSleep]

"sorry for double post"