weak acids/bases

The Conjugate Seesaw

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weak acids/bases

Postby 005199302 » Mon Jan 21, 2019 6:39 pm

If Ka or Kb is given, can we assume that what we're dealing with is a weak acid or base

Reva Kakaria 1J
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Re: weak acids/bases

Postby Reva Kakaria 1J » Mon Jan 21, 2019 6:52 pm

Yes, since strong acids and bases are 100% ionized.

Jennifer Su 2L
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Re: weak acids/bases

Postby Jennifer Su 2L » Mon Jan 21, 2019 6:55 pm

Weak acids have Ka<10^-3 and weak bases have Kb<10^-3. Between two Ka and two Kb, the one with the bigger number is stronger.

Since Ka x Kb = Kw, they are inversely related, which is why a strong base is paired with a weak conjugate acid and a strong acid is paired with a weak conjugate base. This is the "conjugate seesaw." Therefore, you can use this idea to determine which of two acids has a stronger conjugate base, and vis versa.

Ex: (from lecture notes)
Acid 1: Ka=10^-4
Acid 2: Ka=10^-6
Both Acids 1 and 2 are weak acids since they are less than 10^-3, but Acid 2 is weaker than Acid 1 since it is smaller.
Therefore, while both Acids 1 and 2 have strong conjugate bases, the conjugate base of Acid 2 will be stronger.

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