In the textbook it stated "In aqeous solutions, the concentration of hydronium ions is always related to the concentration of hyroxide ions. Therefore we can still measure or calculate the pH of solutions of bases like NaOH, even though they are not acidic." The book also stated that pH is a measure of the concentration of hydonium in the solution. 10^-1M indicates its very acidic. 10^-14 indicates its very basic. OH from NaOH will react with the hyrdonium in the solution only. Just say at the start, in pure water there are 10^-7M of hydronium and hyroxide due to self ionisation. In the end there will be less hyronium left in the solution and how much left will be an indication of the acidity of NaOH? It will depend on how much NaOH is added wouldn't it? The more its added the less hyrodium so the more basic NaOH seems. There will still be 10^-7M of hydroxide ions. But how is the concentration of hydronium related to hyrdoxide ions as the book claims?