Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

The indicator used for titration of weak base and strong acid is (a) thymol blue (b) methyl orange (c) phenolpthalein (d) fluorescein

Step-by-Step Solution

Answer

(b) methyl orange

Step 1: Understanding the Nature of the Titration

In a titration between a weak base and a strong acid, the solution at the equivalence point will be acidic due to the formation of the conjugate acid of the weak base. This means that the pH will be less than 7.

Step 2: Choosing the Appropriate Indicator

The suitable indicator for such a titration should have a pH transition range that falls within the acidic pH range. Methyl orange has a transition range approximately between 3.1 and 4.4, which makes it appropriate for detecting the endpoint of a titration involving a weak base and a strong acid.

Step 3: Comparing Indicator Options

Thymol blue (transition at pH 1.2-2.8 for the acidic range) and phenolphthalein (transition at pH 8.2-10) are not suitable since their transition ranges do not align with the expected acidic pH at the equivalence point. Fluorescein is also not commonly used for this purpose.

Key Concepts

Weak Base

A weak base is a substance that does not fully ionize in solution. This means it only partially accepts protons, resulting in the formation of its conjugate acid.
In the context of a titration, this leads to interesting characteristics:

• The solution does not significantly change pH from neutral when a weak base is dissolved in water.
• At the equivalence point, when exactly the right amount of acid has been added to react with the base, the solution will be slightly acidic rather than neutral, primarily because of the formation of the base's conjugate acid.

Understanding these properties is key when choosing the right indicator for the titration.

Strong Acid

Strong acids are different from weak bases in that they completely ionize in solution. This means they release all of their hydrogen ions, which significantly lowers the pH of the solution. Key features include:

• Strong acids effectively drop the pH significantly, providing readily available hydrogen ions for reaction.
• In a reaction with a weak base, a strong acid will drive the equilibrium to favor the formation of the conjugate acid of the base, resulting in a drop in pH at the equivalence point.

This understanding is crucial for predicting where the pH will fall at different points in the titration process.

pH Indicator

A pH indicator is a chemical compound that changes color depending on the pH of the solution its in. They are employed in titrations to visually indicate the endpoint, which is the point at which the acid and base have completely reacted. The effectiveness of a pH indicator is contingent upon its color change range:

• An effective indicator will have a color change (or transition) range that covers the expected pH at the equivalence point of the titration.
• Choosing the wrong indicator might lead to inaccurate conclusions about the end of the titration, as the color change could occur too early or too late.

This makes understanding the chemical behavior of the solution key to selecting the right indicator.

Methyl Orange

Methyl orange is a type of pH indicator that is particularly valuable for specific titration scenarios, such as those involving a weak base and a strong acid. Why is methyl orange suitable?

• It has a pH transition range between 3.1 and 4.4, which fits within the acidic range expected when a weak base reacts with a strong acid.
• The color change, from red to yellow, is distinct and easily observable, assisting in identifying the endpoint.

Thus, methyl orange is often selected for titration scenarios where the equivalence point pH is below 7, ensuring accurate and clear results.