Question

In the titration of ${\mathrm{CH}}_3\mathrm{COOH}$ against $\mathrm{NaOH}$, which of the following indicator acts as a suitable indicator for this titration?

• A. Methyl red
• B. Methyl orange
• C. Phenolphthalein
• D. Bromothymol blue

Answer 1

The correct option is C

Phenolphthalein

Explanation for the correct answer:-

Option (C) Phenolphthalein

Titration of ${\mathrm{CH}}_3\mathrm{COOH}$ against $\mathrm{NaOH}$:

1. Acetic acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right)$ is a weak acid having a pH value of more than 6 and Sodium hydroxide $\left(\mathrm{NaOH}\right)$ is a strong base.
2. During the titration of ${\mathrm{CH}}_3\mathrm{COOH}$ against $\mathrm{NaOH}$, the addition of ${\mathrm{OH}}^{-}$ ions neutralizes the ${\mathrm{H}}^{+}$ions which are formed by the dissociation of the Acetic acid.
3. Because concentration of ${\mathrm{H}}^{+}$ ion is very low, so on the addition of ${\mathrm{OH}}^{-}$ ions, there would be the increase in pH value.
4. So, the titration range of this reaction lies between $6.5\mathrm{to}10.0$
5. The pH range of phenolphthalein is from $8.2\mathrm{to}10.0$.
6. So, on the addition of phenolphthalein indicator , it undergoes a colour change which proves that the phenolphthalein indicator is suitable for this titration.
7. the complete reaction is:

$\underset{\mathrm{Acetic}\mathrm{acid}}{{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{\mathrm{NaOH}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Sodium}\mathrm{acetate}}{{\mathrm{CH}}_3{\mathrm{COO}}^{-}{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

Explanation for the incorrect answers:-

Option (A):- Methyl red

1. During the titration of ${\mathrm{CH}}_3\mathrm{COOH}$ against $\mathrm{NaOH}$, the titration range lies between $6.5\mathrm{to}10.0$.
2. But, the pH range of Methyl red is from $4.4\mathrm{to}6.2$.
3. So, Methyl red is not the suitable indicator for this titration.

Option (B):- Methyl orange

1. During the titration of ${\mathrm{CH}}_3\mathrm{COOH}$ against $\mathrm{NaOH}$, the titration range lies between $6.5\mathrm{to}10.0$.
2. But, the pH range of Methyl orange is from $3.1\mathrm{to}4.4$.
3. It is a basic indicator.
4. So, Methyl orange is not the suitable indicator for this titration.

Option (D):- Bromothymol blue

1. During the titration of ${\mathrm{CH}}_3\mathrm{COOH}$ against $\mathrm{NaOH}$, the titration range lies between $6.5\mathrm{to}10.0$.
2. But, the pH range of Bromothymol blue is from $6.0\mathrm{to}7.6$.
3. So, Bromothymol blue is not a suitable indicator for this titration.

Therefore, the correct option is (C):- Phenolphthalein.