Question

How do I calculate the Hydronium Ion Concentration of a PH say 12

Answer 1

Several ways ...
One method: [H+] = 10^{-pH}
Then use Kw to find [OH-]

Answer 2

I've tried using [H+] = 10^-12 but I just don't understand how to get it really.

Answer 3

Well that's it, really!

pH and [H+] are two ways of saying the same thing. You don't need very much H+ in a solution to make it acidic (or OH- to make it basic), so the concentrations are going to be very small (something times 10 to a negative exponent).

By taking the -log of the concentration, we get a number that's a bit easier to work with. This is the pH.

So in your example, a pH of 12 indicates an [H+] of 10^(-12) M.