[vlt75]

I would appreciate it very much if someone could point me in the right direction, I'm trying to determine the protic state of phosphoric acid at pH7. I understand that phosphoric acid is a triprotic acid as it has three ionisable hydrogen atoms, which are lost sequentially, but not the slightest clue where to start when working out the above problem!! any help would be great, thanks.! 

[Borek]

Read begining of the page and the last paragraph:

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch