Weak Bases

[Ella Ashley 1D]

This is related to question 10 on the achieve: Why is it that when the pH is below the pKa for a weak base forming its conjugate, the predominant species is charged. I'm mainly confused because for the previous question with a weak acid being ionized, the pH was HIGHER than the pKa and the predominant species was also charged.

[Tyler Ho 1L]

The difference is that #9 deals with a weak ACID and #10 is a weak BASE.

They gave you the pKa of the conjugate acid, which means the K refers to the equilibrium of the "reverse" reaction, where the base is the product and the reactants are the acid and whatever else was on the right. If they gave you pKb, then yes the logic would be the same. Basically, if you look at it like an equilibrium question, which it sort of is, when pH is greater than pKa, whatever the ACID dissociates into is favored (the side without the acid).

pH being greater than pKa does not automatically mean more charged particles; it means more dissociation. If an acid starts charged, then when it deprotonates, it becomes neutral

For #10:
B(aq)+H2O(l)↽−−⇀BH+(aq)+OH−(aq)

BH+ is the acid. So if pH is greater than pKa (the ACID dissociation constant), then the other side without the acid, that being the neutral deprotonated B, will be the predominant species.

[Jennifer Liu 2A]

It may help to rewrite the reaction (products --> reactants) in the future as to visually attempt the problem.

Given that we are given the pKa of the conjugate acid (BH+), we can rewrite the reaction to be:
BH+(aq) + OH-(aq) <--> B(aq) + H2O(l)

Thus, we can use the same visual model the book gives us, in which if pH < pKA, it's "more acidic," and if pH > pKA, it's "more basic."
In this specific question, pH > pKa, so it's "more basic," and the predominant species is neutral.