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Question: To calculate the pH of an acidic buffer solution to which some strong base has been added, how is the concentration of hydronium ions expected to change (resulting in a change in pH) when it seems the only concentrations that are changing are the acid (used up by the hydroxide ions) and conjugate base of the acid (produced by the reaction of the hydroxide ion of the strong base and the acid). How is the concentration of hydronium ions decreasing? Where are these hydronium ions going?
Answer: The amount of H3O+ changes for this reason: You add a strong base, so you are right, the only direct change from that is the change in concentration of acetic acid and acetate. This essentially means you are no longer at K, you are at Q now (changing the concentrations does that to you), where you have decreased the amount of reactant (acetic acid) and increased the amount of product (acetate) (the reactant/product bit assumes you are using the Ka expression of course). So your Q > K, you will use the product to make reactant, meaning that all your reactants concentration (including H3O+) must change (in fact decrease) to get back to K. This makes since in that if you decrease the [H3O+] concentration, the pH should go up after you add the NaOH, which is exactly what it does.
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