pH and pOH

To determine the pH of a solution or the pOH of a solution, you have to know the {H+} concentration or the {OH-} concentration. In order to know how to do this we have to memorize this square

https://strehl-chemistry.wikispaces.com/file/view/Strehl_Square.png/111087617/Strehl_Square.png

An example question one might see includes:
What is the pH of a solution that is 12.5 M HCL?
pH= -log{H+}
     = -log{12.5}
     = -1.097