Problem 39 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

Phenolphthalein (8-9.6) is the best indicator for this titration.

Step 1: Understand the Chemical Reaction

In the titration of a weak acid (like acetic acid) with a strong base (such as sodium hydroxide), the reaction will produce a salt and water. The endpoint of this titration will correspond to the equivalence point where the number of moles of the base equals the number of moles of the acid.

Step 2: Determine the pH at Equivalence Point

For a titration between a weak acid and a strong base, the pH at the equivalence point will be greater than 7. This is due to the hydrolysis of the salt created, which generates excess hydroxide ions ( OH^- ). Typically, the pH will be around 8 to 10.

Step 3: Identify the Suitable Indicator

We need an indicator that changes color at a pH around the equivalence point (pH 8-10). The options are: (a) methyl orange (3-4), (b) methyl red (5-6), (c) bromothymol blue (6-7.5), (d) phenolphthalein (8-9.6). Evidently, phenolphthalein is the most suitable option as its pH range matches well with the anticipated pH at the equivalence point.

Step 4: Conclusion

The best indicator for detecting the endpoint of this titration is phenolphthalein, which changes color in the pH range of 8 to 9.6, aligning with the expected pH range at the equivalence point for a weak acid-strong base titration.

Key Concepts

Weak Acid Strong Base Titration

In a weak acid strong base titration, a weak acid, such as acetic acid, reacts with a strong base like sodium hydroxide. The purpose of this process is to determine the concentration of the acid solution by adding the base solution gradually until the reaction reaches its completion. A titration is not just a random mixing of solutions; it is an accurate and systematic way to find unknown concentrations.

When a strong base is added to a weak acid, the base neutralizes the acid. This interaction leads to the formation of water and a salt. For instance, when acetic acid ( CH_3COOH) reacts with sodium hydroxide ( NaOH), the resulting products are water (H_2O) and sodium acetate ( CH_3COONa).

• Titration accurately measures the volume of base needed to neutralize the acid.
• It helps in calculating the concentration of acetic acid based on the moles of sodium hydroxide used.

Equivalence Point

The equivalence point in a titration is a critical moment. This is when the number of moles of acid equals the number of moles of base. At this point, the acid has been completely neutralized by the base.

In a weak acid strong base titration, even though the acid is fully neutralized, the pH at the equivalence point is not neutral (pH 7). Instead, the pH will typically be greater than 7 due to the nature of the salt formed. For instance, in the reaction of acetic acid with sodium hydroxide, sodium acetate (a salt) is produced. This salt undergoes hydrolysis, which affects the pH.

• Equivalence point is where the neutralization of the acid by the base is complete.
• The pH can be slightly basic due to salt hydrolysis.

Phenolphthalein Indicator

Phenolphthalein is a popular indicator for weak acid and strong base titrations. This is because it changes color within a pH range of 8 to 9.6. This pH range matches well with the expected pH at the equivalence point in these types of titrations.

At lower pH values, phenolphthalein remains colorless. As the pH increases past around 8, it starts turning pink, signaling that the equivalence point is near. This quick shift in color makes phenolphthalein a suitable choice for identifying the titration endpoint. Selecting the correct indicator ensures accuracy in pinpointing the exact moment of neutralization.

• Changes from colorless to pink as pH rises through its range.
• Ensures a clear, visible endpoint.

pH Range

The pH range of a solution is a measure of its acidity or basicity. It is an important aspect of titration as it helps determine which indicator should be used.

In weak acid strong base titrations, the pH of the solution will start on the acidic side due to the weak acid. As the titration progresses and base is added, the pH begins to increase steadily. After the equivalence point, since the solution is primarily of the base excess, it turns more basic. The overall span from initial to final pH gives a complete view of the titration process.

• Initial pH reflects the weakly acidic solution.
• Final pH after the endpoint indicates excess base.

Hydrolysis of Salt

Hydrolysis of salt is a significant concept in titrations of weak acids and strong bases. After the neutralization, the salt formed from the reaction undergoes hydrolysis, which can affect the pH.
For instance, in the titration between acetic acid and sodium hydroxide, the salt sodium acetate is formed. Sodium acetate can react with water in a process called hydrolysis, which increases the concentration of hydroxide ions ( OH^- ). This leads to a solution with a pH higher than 7.

• Salt hydrolysis affects the pH of the solution at equivalence.
• Results in a basic pH due to the formation of OH^- ions.