Acids and Bases Question 11

Data collected during the titration of a 20.0 mL sample of a 0.10 $M$ solution of a monoprotic acid with a solution of NaOH of unknown concentration are plotted in the graph above. Based on the data, which of the following are the approximate p $K_a$ of the acid and the molar concentration of the NaOH?

\begin{array}{c|c|c} & \text{p}K_a & [\ce{NaOH}] \\ \hline \text{A} & 4.7 & 0.050 M \\ \text{B} & 4.7 & 0.10 M \\ \text{C} & 9.3 & 0.050 M \\ \text{D} & 9.3 & 0.10 M \\ \end{array}

Approach

pH is equal to p $K_a$ at the half-equivalence point. On the graph, this corresponds to 4.7.

The titration reaches the equivalence point when 20.0 mL of $\ce{NaOH}$ was added. Since this is equal to the volume of the monoprotic acid, $\ce{NaOH}$ must have the same concentration as the acid.