[Borek]

Dissociation constant is the main one, this plus stoichiometry of dissociation are almost always enough to perform calculations. Still, you need to know what and how. That's described on the page I have pointed to, with a shortcut (in the form of pH cheat sheet) linked at he very bottom of th electure - but don't use it till you will get the basics.

The square root equation can be derived with some sipmilfying assumptions from the dissociation constant; see equation 8.13.