A student titrates 25.00 mL of an HCl solution of unknown concentration with standardized 0.1050 M NaOH using phenolphthalein indicator. The endpoint (faint pink) is reached at 23.42 mL of base. A pH probe is used in a parallel run to record the titration curve, which shows a near-vertical jump centered at pH ~7.
Phenolphthalein has a pKa near 9.4 and changes color over pH 8.2-10.0. Why is it acceptable here even though equivalence is at pH 7?
- Acheck_circle
The strong-strong jump is so steep that the indicator changes within ~0.05 mL of the equivalence point
- B
The titration curve is flat near pH 9, giving a wide endpoint window
- C
Phenolphthalein color change actually occurs at pH 7 in dilute solutions
- D
Indicator pKa does not matter for strong acid - strong base titrations
Explanation
The vertical pH jump from ~3 to ~11 spans phenolphthalein's range; one drop past equivalence brings pH well above 8.2.