A student performs an acid–base titration in which 25.0 mL of 0.100 M HCl(aq) is…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student performs an acid–base titration in which 25.0 mL of 0.100 M HCl(aq) is titrated with 0.100 M NaOH(aq) at 25 °C. The student records the pH continuously as the NaOH is delivered.
Assume that each reagent behaves as a strong electrolyte and that the volume change is the simple sum of the solutions mixed.
(a) Identify the dominant chemical species that determines the pH of the solution before any NaOH is added, and state whether the resulting pH is acidic, neutral, or basic.
(b) When exactly 12.5 mL of NaOH has been added, which reagent is the limiting reactant? Explain how the stoichiometry of the net ionic equation supports your answer without performing a numerical pH calculation.
(c) At the equivalence point, the student observes a rapid change in pH. Describe, in terms of the relative concentrations of [H+] and [OH−], why the pH at the equivalence point is expected to be 7.00 under the conditions of this experiment.
(d) After a total of 37.5 mL of NaOH has been added, explain qualitatively why the pH is well above 7.00 and increases only gradually with further additions of base.
(e) On axes whose horizontal scale represents volume of NaOH added (0 mL to 50 mL) and whose vertical scale represents pH (0 to 14), sketch a qualitative titration curve that is consistent with your explanations in parts (a) through (d). Clearly mark and label the four key regions discussed.