Acids and Bases Question 1

An acetate buffer solution is prepared by combining 50. mL of 0.20 $M$ acetic acid, $\ce{HC2H3O2(aq)}$ , and 50. mL of 0.20 $M$ sodium acetate, $\ce{NaC2H3O2(aq)}$ . A 5.0 mL sample of 0.10 $M$ $\ce{NaOH(aq)}$ is added to the buffer solution. Which of the following is a correct pairing of the acetate species present in greater concentration and of the pH of the solution after the $\ce{NaOH(aq)}$ is added? (The p $K_a$ of acetic acid is 4.7.)

\begin{array}{c|c|c} \hspace{3em}& \text{Acetate Species} & \text{pH} \\ \hline \text{(A)} & \ce{HC2H3O2} & \lt 4.7 \\ \text{(B)} & \ce{HC2H3O2} & \gt 4.7 \\ \text{(C)} & \ce{C2H3O2-} & \lt 4.7 \\ \text{(D)} & \ce{C2H3O2-} & \gt 4.7 \\ \end{array}

Approach

The strong acid and sodium acetate will dissociate completely in water. Acetic acid is a weak acid (hence its p $Ka$ ), and will dissociate slightly. The following reaction will proceed to completion since hydroxide ions will react with any free hydronium ions that dissociate from acetic acid.

\ce{OH- + HC2H3O2 -> H2O + C2H3O2-}

There will therefore be more $\ce{C2H3O2-}$ ions in solution. The pH will be slightly above 4.7 due to the addition of the base.