Question

In base vs acid titration, at the endpoint methyl orange is present as

• A. quinonoid form
• B. heterocyclic form
• C. phenolic form
• D. benzenoid form

Answer 1

The Correct Option is A

To solve this question, we need to understand the chemical behavior of the indicator methyl orange during a base vs acid titration. Let's analyze each option: 

1. Background: Methyl orange is a common pH indicator used in titrations because it changes color at a specific pH level. In acidic solutions, it appears reddish, and in neutral to alkaline solutions, it turns yellow.
2. Endpoint of Titration: The endpoint in a titration is the point at which the reaction between titrant and analyte is complete. For methyl orange, the color change around pH 3.1 to 4.4 is observed due to its structure transformation.
3. Quinonoid Form: At the alkaline endpoint of the titration, methyl orange is present in its quinonoid form, which is responsible for the yellow color. This transformation involves structural rearrangement of its molecular form.
4. Other Forms:
   • Heterocyclic form: Methyl orange contains a heterocyclic moiety in its structure, but this does not define the color change.
   • Phenolic form: This form is not relevant for methyl orange, as its color change doesn't involve phenolic structural patterns.
   • Benzenoid form: Refers to a benzene-like structure and is linked to red color in acidic conditions, not the endpoint color in titration.
5. Conclusion: Thus, at the endpoint of a base vs acid titration, methyl orange is present as the quinonoid form, which results in the characteristic yellow color.

Correct Answer: quinonoid form

Answer 2

Hence at the endpoint, methyl orange is present in quinonoid form.