Question

Identity the titration for which phenolphthalein is suitable indicator.

• A. $HCl$ vs $N{H}_{4}OH$
• B. $C{H}_{3}COOH$ vs $HCl$
• C. $C{H}_{3}COOH$ vs $N{H}_{4}OH$
• D. $KOH$ vs $H_{2}SO4$

Answer 1

Answer: (C), (D)

The correct options are
C $C{H}_{3}COOH$ vs $N{H}_{4}OH$
D $KOH$ vs $H_{2}SO4$

Phenolphthalein changes color at a pH a little above seven. An indicator must change colour at the steepest curve part of any titration curve.
For a titration of strong acid $\left(H_{2}S{O}_4\right)$ with a strong base $\left(KOH\right)$, the titration curve is steepest at $pH=7$. However, it is still very steep either side of this, and phenolphthalein indicates the color change.
For a titration of weak acid $\left(C{H}_{3}COOH\right)$ against strong base $\left(N{H}_{4}OH\right)$, the titration curve is steepest a little above $pH=7$ . Hence, phenolphthalein does the job.