Predominant Species Present at a Certain pH

[Dahlia Kirov 3A]

Hi!

I am really confused about where to start / how to work through the kind of question that asks whether the predominant species of a reaction present at a certain pH is neutral or charged. I believe we have to compare the pH to the pKa value since both are given in the problem, but I don't know what to infer from the comparison. I attached an example below if that helps with explaining...

B(aq)+H2O(l)↽−−⇀BH+(aq)+OH−(aq)

where
B is a weak base
pKa = 8.07
pH = 7.19

predominant species present at pH 7.19 = ?

[shonanchiang2L]

Hello! If pH < pKa, the compound will stay in its protonated form. In the example you wrote, pH < pKa, so the predominant species will be BH+ because that is the form with the H+ on it, and the predominant species will therefore be charged. I hope this helps!

[Alyssa_Miao_3B]

I think the way to approach these kinds of problems is to just memorize the general rule. For acids, when pH > pKa, the charged species (A-) is predominant. When pH < pKa, the neutral species (HA) is predominant. For bases, when pH > pKa, the neutral species (B) is predominant. When pH < pKa, the charged species (BH+) is predominant. In the question you put, pKa > pH and we are looking at a base, so this would mean that the charged species (BH+) would be predominant.