pH = -log[H3O+]

where [H3O+] is the concentration of hydronium ions in moles per liter (M).

In this case, you've given the concentration of H3O+ as 1 x 10^-5 M. Let's plug that into the formula:

pH = -log(1 x 10^-5)

pH = 5

Therefore, the pH of the solution is 5.