Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenolphthalein acts as an indicator within a \(\mathrm{pH}\) range of (a) \(8.2-10\) (b) \(6.2-7.2\) (c) \(4.2-6.2\) (d) \(2.2-4.2\)

Step-by-Step Solution

Answer

Option (a) 8.2-10 is the correct pH range for phenolphthalein.

Step 1: Identify the pH range for phenolphthalein

Phenolphthalein is a commonly used acid-base indicator that changes color at certain pH levels. It is colorless in acidic solutions and pink in basic solutions. The recognized pH range for phenolphthalein to change color is between pH 8.2 to 10.

Step 2: Compare with given options

The problem provides several pH ranges to choose from: (a) 8.2-10, (b) 6.2-7.2, (c) 4.2-6.2, and (d) 2.2-4.2. Compare these options with the known pH range where phenolphthalein acts as an indicator.

Step 3: Match the correct range

Among the given options, only option (a) 8.2-10 matches the known range for phenolphthalein. This means that phenolphthalein acts as an indicator within this specific pH range. The other options are outside the effective range for phenolphthalein.

Key Concepts

Phenolphthalein

Phenolphthalein is a fascinating chemical compound commonly used as a pH indicator in various types of laboratory experiments. It is an organic dye that undergoes a noticeable color change depending on the acidity or basicity of the solution it is in. In acidic conditions, phenolphthalein remains colorless, providing a clear and transparent look. However, when the solution turns basic, specifically when the pH exceeds 8.2, phenolphthalein transforms into a vibrant pink color. This deep pink color persists up to a pH of around 10. Beyond this range, the color intensity may not change further, designating this range as optimal for detecting shifts between acidic and basic states.

Knowing how and when phenolphthalein changes color is crucial in titrations and other chemical reactions. It helps chemists visually determine the endpoint of a reaction, making this compound indispensable in laboratories. Its unique color change property serves as a powerful tool for students and professionals to understand and visualize the behaviors of acids and bases in practice.

Acid-Base Reactions

Acid-base reactions are a type of chemical reaction involving the exchange of protons (H⁺) between an acid and a base. These reactions are fundamental in chemistry and have wide-reaching implications in various scientific fields and everyday life. An acid typically donates a proton, resulting in the formation of its conjugate base, while a base accepts a proton, forming its conjugate acid.

A classic example of an acid-base reaction is the neutralization process, where an acid and a base react to form water and a salt. For instance, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), water and sodium chloride (salt) are produced. The reaction is generally represented as:

• Acid + Base → Water + Salt

These reactions often feature pH changes, which are discernible using indicators like phenolphthalein.

Understanding acid-base reactions is crucial for grasping the basics of pH, electrolytes, and many biological processes that are essential for life. These reactions demonstrate how substances interact at a molecular level and are a key component of comprehending chemical equilibria and thermodynamics.

pH Range Determination

Determining the appropriate pH range is essential for utilizing indicators like phenolphthalein effectively. To understand the functioning of such indicators, one must first comprehend what pH represents.

pH is a scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14, with 7 being neutral, below 7 indicating acidity, and above 7 indicating basicity. pH is logarithmic, which means that each whole number change on the scale represents a tenfold change in acidity or basicity.

With this understanding, the concept of the pH range comes into play when using indicators. For phenolphthalein, the effective pH range of 8.2 to 10 means it will clearly signal a transition from an acidic or neutral to a more basic environment by changing to pink. This characteristic is particularly useful in titration procedures, where accurately identifying the neutralization point is desired.

Knowing the pH range where a particular indicator operates aids in choosing the right one for experiments and guarantees accurate and meaningful results. Evaluating this properly ensures that chemical reactions are monitored precisely, providing critical insights for both educational and professional chemical analysis.