Answer
$pK_a$ is defined as the negative logarithm of the $K_a$ value.
$pK_a = -log({K_a}_{})$
The $pK_a$ is inversely proportional to the strength of the acid; therefore, the stronger the acid, the lower its $pK_a$ value.
Work Step by Step
As we know, the $K_a$ is proportional to the acid's strength.
Since we have a "-" on the $pK_a$'s formula, that relation is inverted, so the $pK_a$ is inversely proportional to the strength of the acid.
