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Difficulty: MediumAcid-Base Titrations, Indicators, and Volumetric Calculations

Complete the statement regarding the choice of indicator and the expected endpoint color change when titrating aqueous ammonia with hydrochloric acid.

Answer:When titrating aqueous ammonia against hydrochloric acid, the appropriate indicator is 【methyl orange】, which changes from yellow in alkaline solution to 【red】 at the equivalence point.

Answer

The appropriate indicator is methyl orange, which changes from yellow to red (or pink) at the equivalence point.
In a titration between a strong acid (HCl\text{HCl}) and a weak base (NH3\text{NH}_3), ammonium chloride (NH4Cl\text{NH}_4\text{Cl}) is formed. The hydrolysis of NH4+\text{NH}_4^+ ions makes the solution acidic at the equivalence point (pH<7\text{pH} < 7). Methyl orange is the correct indicator because its transition range (pH 3.1−4.4\text{pH } 3.1 - 4.4) coincides with this acidic equivalence point, turning red (or pink) when the endpoint is reached.

Step-by-Step Solution

1
Analyze the nature of the reactants
Hydrochloric acid (HCl\text{HCl}) is a strong acid, and aqueous ammonia (NH3(aq)\text{NH}_3(aq)) is a weak base.
The relative strengths of the acid and base determine the pH at the equivalence point.
2
Determine the pH range at the equivalence point
The salt formed (NH4Cl\text{NH}_4\text{Cl}) undergoes hydrolysis, producing an acidic solution with an equivalence point at pH<7\text{pH} < 7 (typically between pH 3\text{pH } 3 and 66).
Salt hydrolysis of a strong acid and weak base yields ammonium ions (NH4+\text{NH}_4^+) which hydrolyze to produce H3O+\text{H}_3\text{O}^+ ions.
3
Select the suitable indicator and recall its color change
Methyl orange functions in the pH\text{pH} range of 3.1−4.43.1 - 4.4, changing from yellow in basic solution to red (or pink) at the acidic endpoint.
An indicator must undergo a distinct color transition within the steep pH\text{pH} region of the titration curve near the equivalence point.

Key Concept

Indicator Selection and Endpoint Determination in Volumetric Analysis
Estimated Time:1m 0s
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Topics

Acid-Base Titrations, Indicators, and Volumetric Calculations

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