A 25.0 mL sample of an aqueous monoprotic weak acid is titrated with 0.100 M NaOH
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 25.0 mL sample of an aqueous monoprotic weak acid is titrated with 0.100 M NaOH. The instructor has provided a written description of the pH–volume titration curve:
• Before titrant is added, the solution pH is 2.88.
• From 0 mL to about 5 mL of NaOH added, the curve rises slowly, forming a noticeably flat region.
• Near 12.5 mL of NaOH added, the pH is 4.76.
• Between 20 mL and 30 mL added, the curve becomes very steep; the point of greatest slope occurs at 25.0 mL.
• At 25.0 mL added, the pH is 8.70.
• Beyond 30 mL added, the pH increases gradually and approaches 12.3 when 35.0 mL of NaOH have been added.
Use this information to analyze the titration curve for the acid.
(a) Identify the four distinct regions of the curve (initial, buffer, inflection, and post-equivalence) and state the approximate titrant-volume interval that corresponds to each region.
(b) Explain, at the particulate level, why the pH remains nearly constant in the buffer region even though hydroxide ions are being added continuously.
(c) Account for the fact that the pH at the equivalence point is greater than 7.00.
(d) Describe how the numerical value of the acid dissociation constant Ka could be obtained directly from information that is already provided in the description.