Achieve Week 10 #12

[Adele Nguyen 2G]

A monoprotic weak acid, HA , is ionized according to the reaction

HA(aq)+H2O(l)↽−−⇀A−(aq)+H3O+(aq)pKa=3.58

where A− is the conjugate base to HA.
For this weak monoprotic acid, the predominant species present at pH 1.81 is

unknown.

neutral.

charged.

Can someone explain to me why the answer is neutral?

[Sujin Lim 1H]

Since this monoprotic weak acid is in a solution that is more acidic than it, we know that the solution must have a stronger acid than this monoprotic weak acid (if the pKa of the acid is larger than the pH of the solution, it means the solution is more acidic than the monoprotic weak acid). Therefore, the strong acid in the solution will want to give off its H+ ions more easily than the weak acid, keeping the weak acid protonated. If the solution is already more acidic, there is no reason for the acid to be giving off its proton.
On the other hand, if the weak acid was in a solution where the pH of the solution was higher than that of the pKa of the weak acid, the weak acid will then be in a negatively charged state because it will be the stronger acid (than the acid in the solution), thus giving off its proton and becoming negatively charged. Hope this helps!

[Austin_Schwartz_1L]

Hello Adele,
When looking at conjugate bases, if the pKa is less than the pH, the predominant species is charged. If the pKa is greater than the pH, it is neutral. Finally, if the pKa is equal to the pH, we are unable to tell. In this example, the pKa is greater than the pH, which means that the predominant species will be neutral.
I hope this helps!