Phenolphthalein is commonly used to visualize the progress of titrations involving…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Phenolphthalein is commonly used to visualize the progress of titrations involving weak acids and strong bases. Relevant information is provided below.
• pKa=9.4
• Acid form: colorless
• Base form: pink
A student titrates 25.0 mL of a monoprotic weak acid (initial pH = 2.0) with 0.100 M NaOH. The student records the pH at several key stages, summarized in the table.
Stage | Volume of NaOH added (mL) | Measured pH
Initial | 0.0 | 2.0
Half-equivalence | 12.5 | 4.8
Equivalence | 25.0 | 9.8
Past equivalence | 30.0 | 11.0
Using only the information above, analyze how the appearance of phenolphthalein changes during the titration.
(1) For each of the four stages listed, determine which structural form of phenolphthalein (acidic or basic) is present in greater concentration. Support each determination with appropriate reasoning based on pH relative to pKa.
(2) For each stage, state whether the indicator solution will appear colorless, faint pink, or strongly pink. Explain how the microscopic composition at that stage leads to the observed macroscopic color intensity.
(3) On a set of labeled axes, sketch a qualitative graph of indicator color intensity (vertical axis) versus volume of NaOH added (horizontal axis). Clearly indicate where significant color change begins and where it levels off. Briefly narrate your sketch to connect it to the reasoning in parts (a) and (b).