Answer
$K_a$ = 55.55 M
$pK_a$ = -1.74
Work Step by Step
The equilibrium for hydronium ion is written in the picture. $H_3O^+$ is the acid and $H_2O$ is the conjugate base. Using the $K_a$ formula, and substituting the acid and conjugate base, we get [H_2O]. Then using the formulas shown in the picture, we can find the $K_a$.
$pK_a$ = -log($K_a$) = -1.74
