Ibuprofen is an acid with a pKa of 4.9; its structure includes a carboxyl group. To be absorbed into the bloodstream, it must pass through the membrane lining the stomach and the small intestine. Electrically neutral molecules can pass through a membrane more easily than can charged molecules. Would you expect more ibuprofen to be absorbed in the stomach, where the pH of gastric juice is about 1, or in the small intestine, where the pH is about 6