The indicator used in a titration between strong acid and weak base is

Analysis

The question asks for the appropriate chemical indicator to use when performing a titration between a strong acid and a weak base.

Concept Explanation

In acid-base titrations, the choice of indicator depends on the pH range at which the indicator changes color (the "pH range of the indicator") and the pH at the equivalence point of the reaction.

1. Strong Acid + Strong Base: The equivalence point is at pH 7. Any indicator (like phenolphthalein or methyl orange) works.
2. Strong Acid + Weak Base: The resulting salt is acidic because the conjugate acid of the weak base hydrolyzes in water. Therefore, the equivalence point occurs at a pH lower than 7 (typically between pH 3 and 5). We need an indicator that changes color in this acidic range.
3. Weak Acid + Strong Base: The equivalence point is at a pH higher than 7 (typically between 8 and 10). Phenolphthalein is the standard choice here.

Step-by-Step Solution

• Step 1: Identify the nature of the reaction. A strong acid (e.g., HCl) reacting with a weak base (e.g., $NH_3$) produces an acidic salt.
• Step 2: Determine the pH at the equivalence point. Because the acid is strong and the base is weak, the solution will be acidic at the endpoint (pH < 7).
• Step 3: Evaluate the indicators: • Methyl orange: Changes color in the pH range of 3.1 – 4.4. This matches the acidic equivalence point of a strong acid-weak base titration. • Phenolphthalein: Changes color in the pH range of 8.2 – 10.0. This is used for weak acid-strong base titrations. • Bromothymol blue: Changes color around pH 6.0 – 7.6, which is better suited for strong acid-strong base titrations.

Since methyl orange changes color in the acidic range, it is the correct choice for this titration.

Exam Tip

Remember the mnemonic "SAWB-MO":

• Strong Acid + Weak Base = Methyl Orange.
• Weak Acid + Strong Base = Phenolphthalein (WASP).