Question

Given below are two statements :
Statement I : In the titration between strong acid and weak base methyl orange is suitable as an indicator.
Statement II : For titration of acetic acid with NaOH phenolphthalein is not a suitable indicator.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

SA + WB $\rightarrow$ Acidic pH $\rightarrow$ Methyl Orange.
WA + SB $\rightarrow$ Basic pH $\rightarrow$ Phenolphthalein.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The choice of an indicator in an acid-base titration depends on the pH at the equivalence point. An indicator is suitable if its color change interval (pKa $\pm$ 1) overlaps with the sharp pH change at the equivalence point.
Step 2: Detailed Explanation:
1. Analysis of Statement I: In a titration between a strong acid (SA) and a weak base (WB), the resulting salt undergoes hydrolysis, making the solution slightly acidic at the equivalence point ($pH<7$). Methyl orange has a working pH range of 3.1 to 4.4, which falls within the acidic range. Thus, it is a suitable indicator. Statement I is true.
2. Analysis of Statement II: Titration of acetic acid (Weak Acid, WA) with NaOH (Strong Base, SB) results in a basic salt (sodium acetate). At the equivalence point, the $pH>7$ due to anion hydrolysis. Phenolphthalein has a working pH range of 8.3 to 10.0, which perfectly overlaps with this basic equivalence point. Therefore, it is a suitable indicator. Statement II is false.
Step 3: Final Answer:
Statement I is true but Statement II is false.