Description: Aspirin has been around for over 100 years. Like many NSAIDs it con
ID: 1076702 • Letter: D
Question
Description: Aspirin has been around for over 100 years. Like many NSAIDs it contains a carboxylic acid. This is interesting to us because that means it can exist in two different forms, as a carboxylic acid at low pH and as a carboxylate at high pH.
Instructions: Your job for this case study is to investigate the pKa of aspirin’s carboxylic acid. Once you have that number in hand compare it to the pH of both the stomach (use the middle of the stomach pH range) and the small intestine (again assume the middle of the range). With all these numbers in hand, estimate the ratio of carboxylic acid to carboxylate for aspirin in both the stomach and the small intestine. Please show your work.
Explanation / Answer
Pka value of aspirin is 3.5, pH of stomach is 1.5 to 3.5, so mid value is 2.5, pH of small intestine is 6 to7.4 so mid value is 6.7
pH = pka + log(Conjugate base/acid)
For stomach
2.5= 3.5 +log( carboxylate/carboxyllic acid)
Carboxylate/carboxyllic acid = .0316
Carboxyllic acid/carboxylate = 31.64
For small intestine
6.7= 3.5 + log(carboxylate/ carboxyllic acid)
Carboxylate/ carboxyllic acid = 1584.89
Carboxyllic acid/carboxylate =6.3 * 10^(-4)