Achieve Question 13

[Angela_Li_1J]

In this question, it asks what the predominant species present at a pH of 5.55 is for a weak monoprotic acid. The pKA is 4.32. How would I approach this problem?

[Molly McAndrew 1 1H]

You need to remember these 3 rules:

If the pH of a solution is equal to the pK_a of the weak acid, there are equal amounts of the neutral, protonated species (in this case, HA) and the charged, deprotonated species (in this case, A^-) in the solution.

If the pH of a solution is higher than the pK_a of the weak acid, the charged, deprotonated species (in this case, A^-) will become more predominant in the solution.

If the pH of a solution is lower than the pK_a of the weak acid, the neutral, protonated species (in this case, HA) will become more predominant in the solution.

So, in this case, the pH is lower than the pK_a. Therefore, the neutral HA species will be more predominant.