To calculate the pH of a solution, we can use the formula:
pH = -log[H3O+]
where [H3O+] is the concentration of hydronium ions in the solution.
Substituting the given value of [H3O+] into the formula, we get:
pH = -log(8.5 x 10^-5) = 4.07
Therefore, the pH of the solution is approximately 4.07.