Calculating pH of solutions?

Why must the self-ionization equilibrium be taken into account for solutions of sodium hydroxide at higher concentrations when calculating the pH?

• This is specifically about aqueous solutions of sodium hydroxide. How does the self-ionization equilibrium have anything to do with calculating the pH when sodium hydroxide is concentrated in water? When does the self-ionization equilibrium (specifically, of water) not have to be taken into account when calculating the pH of acidic/alkaline solutions and why?

• Answer:

  For strong acids at high concentration, you can directly calculate the H+ concentration since it will be "identical" to the concentration of the acid. In all other cases you need to know the water constant. Your main question is a perfect example of this: if you dissolve 1 mole of sodium hydroxide to a liter of solution, you know that the OH- concentration is 1 mole per liter as well. Or pOH=0. So how can you calculate what the pH is? You need to use the equality that pH+pOH=14, which is another way to write the water constant.