Chapter U6, Problem 2STP

Interpretation Introduction

Interpretation:

The form (ionized or unionized) of the indicator present in a solution based on the color of the solution needs to be determined.

Concept introduction:

An acid-base indicator is a chemical compound that is added in small proportions to a titration mixture and indicates the pH of the mixture visually. An ideal acid-base indicator usually has two distinct colors, one in the acid medium and the other in the basic medium. The color of the indicator indicates whether the titration mixture is acidic or basic in nature. Thus, the acid-base indicator is a chemical indicator for proton, H⁺. Depending on a high or low concentration of H⁺, the indicator exhibits different colors.

Answer to Problem 2STP

Option (D) is the correct answer.

Explanation of Solution

Reason for Correct Option:

The acid-base indicator HIn has distinct colors in the protonated (acid) and deprotonated (basic or In^-) form. The protonated indicator HIn is red while the deprotonated indicator, In- is yellow. The addition of the indicator to the solution produces an orange color.

It is common knowledge that orange is an intermediate color between yellow and red. Thus, the indicator exists as a mixture of both the protonated and the deprotonated forms, i.e., the solution contains both In- anions and HIn molecules.

Conclusion

Reasons for Incorrect Options:

Since the solution doesn’t exhibit the color of the pure acid form (HIn) or the pure basic form (In^-), hence, the solution contains a mixture of both the species and thus, the options (A), (B) and (C) are incorrect.