Achieve questions 9 and 10

[805939635]

Questions 9 and 10 on achieve assignment #2 ask to find whether the predominant species is charged or neutral based on pH and pKa. I am not sure how to find this. I understand there's a relationship that depends on whether the pH or the pKa is higher, but I don't know which of those means which or why.

[Jenny Tran 2B]

hi! from my understanding, when the pH of the solution > the pKa of a weak acid, the charged species is predominant. and when the pH of the solution < pKa of a weak acid, the neutral species is predominant. however, when the pH of the solution > pKa of the conjugate acid, the neutral species is predominant. and when the pH of the solution < pKa of a conjugate acid, the charged species is predominant.

[Matthew Morita 2F]

Hello,
So the reason why it has to do with pH and pKa was explained in one of the workshops I went to. For example, pH<pKa. If you simplify this into logs then -log[H+]<-log[Ka]. Then you can cancel the -log on each side and since you are dividing by a negative you change the sign. Therfore [H+]>Ka. You can simply this into [H+]>{A-]*[H+]/[HA]. You can cancel the [H+] on each side which makes the equation 1>[A-}/[HA]. Then you multiply each side by [HA-] which gets you [HA]>[A-]. Therefore, if pH<pKa there will be more [HA] and the solution will be neutrally charged. This is for acids but im sure the math is relatively the same for bases as well. Hope this helps