Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

The best indicator for detection of end point in titration of a weak acid and strong base is (a) methyl red ( 5 to 6 ) (b) methyl orange ( 3 to 4 ) (c) phenolphthalein \((8\) to \(9.6)\) (d) bromothymol blue ( 6 to \(7.5\) )

Step-by-Step Solution

Answer

The best indicator is phenolphthalein (8 to 9.6).

Step 1: Understanding the Problem

We are asked to find the best indicator for detecting the endpoint in a titration involving a weak acid and a strong base. This type of titration typically results in an equivalence point in the basic pH range.

Step 2: Identify the Expected pH Range

For titration of a weak acid with a strong base, the pH at the equivalence point is greater than 7 because the conjugate base of the weak acid remains in the solution and tends to make it basic.

Step 3: Matching pH Ranges with Indicators

We compare the endpoint pH to the given pH ranges for each indicator: - Methyl Red (5-6) - Methyl Orange (3-4) - Phenolphthalein (8-9.6) - Bromothymol Blue (6-7.5). The pH range should ideally overlap the pH of the expected endpoint.

Step 4: Selecting the Appropriate Indicator

Since the equivalence point for the titration of a weak acid with a strong base is above pH 7, phenolphthalein, with a pH range of 8 to 9.6, is the best choice to match this basic pH range.

Key Concepts

Understanding Acid-Base Indicators

Acid-base indicators are special chemicals used to identify the endpoint of a titration, which is the moment when the acid and base have completely reacted. These indicators change color depending on the pH of the solution, making them valuable tools in various chemical experiments.
Understanding how these indicators work can help you choose the right one for specific titrations.

• Color Change: Each indicator has a specific pH range where it changes color. This range indicates the acidity or basicity at which the indicator undergoes a visible color change.
• Importance: An indicator must have a pH range that matches the expected pH of the titration's equivalence point, where equal amounts of acid and base have been combined.

Choosing the correct indicator ensures accurate determination of the titration endpoint.

The Role of Phenolphthalein in Titrations

Phenolphthalein is a widely used acid-base indicator, especially in titrations involving a weak acid and strong base. This is because it has a pH range between 8 and 9.6, which concurs with the basic pH environment typically found at the equivalence point in such titrations.

• Characteristics: Phenolphthalein is colorless in acidic solutions but turns pink to magenta as the solution becomes more basic.
• Usage: Its clear color change makes it ideal for experiments where precision is key, such as detecting the endpoint in a titration.

By using phenolphthalein, chemists can accurately pinpoint when enough base has been added to neutralize the weak acid, leading to more precise and reliable outcomes in the laboratory.

Exploring Weak Acid-Strong Base Titrations

A weak acid-strong base titration is a common laboratory technique used to study acid-base reactions. In this process, a strong base is gradually added to a weak acid until neutralization occurs.
The unique aspect of this type of titration is the resulting pH level at the equivalence point.

• Equivalence Point: Since the weak acid doesn't completely dissociate, its conjugate base remains in the solution, making the equivalence point basic. This generally raises the pH above 7.
• Indicator Selection: Due to the basic nature of the equivalence point, choosing an indicator like phenolphthalein, which changes color at higher pH levels, is crucial for accuracy.

Understanding such titrations is essential for correctly interpreting results and conducting successful experiments that involve weak acids and strong bases.