Let's say I have a buffer made of 0.5M acetic acid and 0.5M potassium acetate. I let this solution react until equilibrium, where I can calculate the pH, etc. Now, I add some NaOH to it. The method that my textbook uses to calculate the new pH, is to essentially separate the question into two parts. First considering the reaction between hydroxide and acetic acid, in which all the hydroxide is reacted. (Amount of acetic acid remaining being 0.5M - amount of hydroxide added) Then, considering the equilibrium reaction with the new values of concentration of acetic acid, and then calculating the Hydronium ion concentration, and therefore, pH.
What's confusing me, is that doesn't this in a way, not make sense? The method is essentially treating the problem as if I added sodium hydroxide along with the acetic acid and potassium acetate at the same time, as opposed to having the buffer first, then adding the sodium hydroxide. Wouldn't the hydroxide ions react with the Hydronium ions (in the buffer), instead of the acetic acid? And even even if the hydroxide ion reacts with the acetic acid instead, why is it using the original concentration of acetic acid in the calculation of the reaction with hydroxide? Wouldn't the actual concentration of acetic acid be smaller than the original, since the solution went to equilibrium first, before any sodium hydroxide was added?