## Organic Chemistry 9th Edition

An acid with a small $pK_{a}$ value or a large $K_{a}$ value is a strong acid. A weaker acid, in comparison, has a smaller $K_{a}$ value or a larger $pK_{a}$ value. $pK_{a}$ of Phenylalanine = 1.83 $pK_{a}$ of Tryptophan = 2.83 smaller value of $pK_{a}$ = 1.83 (for Phenylalanine) and hence it is stronger of the two.