I have now calculated my pKa value. It is 0,2 % deviant(spelling?) from what the assignment want me to compare it to. I have also calculated the [OH-] using the [H+].

I think i have understood what this assignment is about and how to calculate the problems, it has a lot to do with the help you have given me, thank you very much for your time and effort in helping me understand. It is really appreciated!