[Feynman95]

Hello, I understand that the general procedure to find the PH of a strong base is to assume the OH- concentration is equal to the concentration of the base, then use kw to find [H+], and then use PH=-logH+.

However for Kw to be constant the equilibrium for the dissociation of water has to shift to the left when the base is added due to all the extra OH- ions that are released. If this is the case, then you cannot then assume that the OH-concentration is that of the original base. Is there something i'm missing?

Thank you

[Borek]

If I understand correctly what you mean - there is a huge excess of the OH^-, so you can safely ignore changes due to the water autodissociation. It starts to play a role when the base concentration gets below 10^-6 M (or even 5×10^-7 M).