Answer
The stronger acid is Phenylacetic acid. It is 3.9 stronger than propionic acid.
Work Step by Step
To calculate how much stronger it is, the following calculation is used:
$K_a$ phenylacetic acid/ $K_a$ propionic acid =
5.2 x $10^{-5}$/ 1.35 x $10^{-5}$ = 3.9
Phenylacetic acid is 3.9 times stronger than propionic acid.