to the pH of weak base or weak acid suppose :

B + H

_{2}O

BH + OH

^{-} here we can determine the pH by calculating [OH

^{-}] by the following equation

K = X

^{2} / 1-X

where X is the concentration of OH

^{-} ion & K is the dissociation constant of the base

by solving the euation you can determine the pH

the final thing here is that the concentration of water is ignored as it is the general solvent found every where in the solution so its voncentration is ignored