Aspirin is a weak acid at low pH and has a pKa of 3.5. Per unit surface area, will more aspirin be absorbed in the stomach (pH 1.5) or in the intestine (pH 6.5)?
At a pH below its pKa, aspirin will be in its acid form (RCOOH). At a pH above its pKa, aspirin will be in its base form (RCOO-). Since non-ionized molecules cross cell membranes passively whereas ionized molecules do not, more aspirin will be absorbed in the stomach than in the intestine