Salicylic acid, HOC6H4CO2H, and its derivative have been used as pain reliever for a long time. Salicylic acid occurs in small amount in the leaves, bark, and roots of some vegetation (most notably historically in the bark of the willow tree). Extracts of this plant have been used as medication for century. The acid was first isolated in the laboratory in 1838.
(a) Both functional groups of salicylic acid ionize in water, with Ka = 1.0 × 10−3 for the—CO2H group and 4.2× 10−13 for the −OH group. What is the pH of a saturated solution of the acid (solubility = 1.8 g/L).
(b) Aspirin was discovered as a result of efforts to produce a derivative of salicylic acid that would not be irritating to the stomach lining. Aspirin is acetylsalicylic acid, CH3CO2C6H4CO2H. The −CO2H functional group is still present, but its acidity is reduced, K a = 3.0 × 10−4. What is the pH of a solution of aspirin with the same concentration as a saturated solution of salicylic acid (See Part a).