A woman is brought into the emergency room experiencing nausea and vomiting. She is also hyperventilating. She says that she ingested a bottle of aspirin about two hours earlier. Aspirin (acetylsalicylic acid) is converted to salicylic acid in the stomach. Assuming 100% conversion to salicylic acid. Required:
Calculate the percentage of protonated and unprotonated forms of salicylic acid in her stomach, given a pH of 2.0 for the stomach.

Protonated form: 96.87%

Unprotonated form: 3.13%

Explanation:

To find the percentage of protoned and unprotoned forms of salicylic acid we must use Henderson-Hasselbalch equation:

pH = pKa + log [A⁻] / [HA]

Where pKa of salicylic acid is 3.49; [A⁻] is the unprotonated form and [HA]:

2.0 = 3.49 + log [A⁻] / [HA]

0.03236 = [A⁻] / [HA] (1)

And as:

[A⁻] + [HA] = 100% (2)

Replacing (2) in (1):

0.03236 = 100 - [HA] / [HA]

0.03236 [HA] = 100 - [HA]

1.03236[HA] = 100

And [A⁻]:

[A⁻] = 100 - 96.87