To determine the pH of an acid we must first determine if it is a weak acid or a strong acid. A strong acid will completely dissociate in water. This allows us to determine the concentration of the hydronium ion created when mixed with water as the concentration of hydronium ions will be the same as the concentration of the acid. If it is a weak acid, it will not dissociate completely and the concentration will need to be determined from the equilibrium equation.
To determine the strength of the acid we need to know the pKA. The easiest way to do this is to find it in a table of acids and bases. If the pKA is greater than 1.00 then we know that acid completely dissociates and is therefore a strong acid. the pKA for HClO4 (perchlorate acid) is 3.2x10^9, thus perchlorate is a very strong acid. Therefore the [H3O+] (concentration of hydronium ions) is the same as the concentration of the acid solution.
We can now use the definition of pH to find our answer
pH = -Log[H3O+] = -Log(7.9x10^-9) = 8.1