Sapling Homework 2, Question 9

[Bella Wachter 1A]

While completing the Week 2 Sapling Homework, the explanations behind question 9 gave me some pause, and I was hoping someone could help me make sense of the answer explanations.

It says that for weak acids, when the acid's pKa is equal to the pH, the concentration of the weak acid is equal to the concentration of its conjugate base. This logically makes sense to me. However, the next explanation got a bit confusing.

How is it that when the pH is below the pKa, that HA (the undissociated acid) is more predominant in the solution? Shouldn't lower pH values correspond to a higher conjugate base concentration instead, since a lower pH means a higher H3O+ concentration (and H3O+ concentration = conjugate base concentration)?

[Rachel Jiang 3H]

I think an easier way to think of this question is that if the pH is lower than the pKa of the acid, this means that the environment is acidic. If the acid is already under acidic conditions, then it would not be deprotonated to produce more H3O^+ ions which would lower the pH even further. Therefore, when pH<pKa, the undissociated acid HA would be predominant.

[Gillian Gabrielsen2F]

I view it as when pH<pKa means that the environment has a stronger acid than HA. This means when HA dissociates into A-, the stronger acid protonates A- ions, leading to there being more HA in the solution. Since HA isn't a stronger acid than the environment, it's unable to further lower the pH.