What is the pH of a solution having a H3O+ concentration of 3.50 X 10-5 M?

Answer 1

pH = -log( [H⁺] )

For conventional purposes, H₃O⁺ and H⁺ refer to the same ions, given you are working in an aqueous solution.

pH = -log(3.50x10^-5 M)

pH = 4.456

pH does not have units, since it represents a unitless exponent. Your pH will also have one more significant figure than your concentration, for mathematical reasons I won't get into.

In general, you will need a scientific calculator to easily answer this type of question.