A solution is prepared by adding 100 mL of 1.0 \(M \ce{HC2H3O2(aq)}\) to 100 mL of 1.0 \(M \ce{NaC2H3O2}\). The solution is stirred and its pH is measured to be 4.73. After 3 drops of 1.0 \(M \ce{HCl}\) are added to the solution, the pH of the solution is measured and is still 4.73. Which of the following equations represents the chemical reaction that accounts for the fact that acid was added but there was no detectable change in pH?

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\(\ce{NaC2H3O2}\) acts as a buffer. It will first dissociate in solution:

$$ \ce{NaC2H3O2(aq) -> Na+(aq) + C2H3O2-(aq)} $$

Excess hydronium ions from \(\ce{HCl}\) react in the following way with acetate (\(\ce{ C2H3O2-}\)) ions:

$$ \ce{H3O+(aq) + C2H3O2-(aq) -> HC2H3O2(aq) + H2O(l)}$$

The pH of 1 \(M\) acetic acid \((\ce{HC2H3O2}\)) is approximately 4.73. Only a few drops of strong acid were added. The reaction above effectively adds a small amount of acetic acid and is unlikely to significantly change the pH.