What does pH measure?
Water is known to undergo autoprotolysis according to the following reaction:
This equilibrium has been exhaustively studied and we write the equilibrium in the normal way,
A temperature is specified, because the extent of reaction depends upon temperature, especially for a bond-breaking reaction. Now this is a mathematical expression, the which we can divide, multiply etc., provided that we do it to BOTH sides of the expression. One think we can do is to take
Of course, by definition
So for water at
So to answer your question (finally!),
I apologize for going on so long, but you will need the given background if you don't know it already.