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

Complete the statement regarding indicator color changes during an acid-base titration by filling in the blanks.

Answer:When titrating ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) with sodium hydroxide (NaOH\text{NaOH}) using phenolphthalein as the indicator, the solution in the conical flask changes from 【colorless】 to 【pink】 at the end point.

Answer

The solution in the conical flask changes from colorless (or colourless) to pink at the end point.
Ethanoic acid is a weak acid and sodium hydroxide is a strong base. The equivalence point occurs above pH 7 in the alkaline region. Phenolphthalein is the most suitable indicator for this titration because its color transition range is pH 8.3 to 10.0. Initially, the solution in the conical flask is acidic, rendering phenolphthalein colorless. At the end point, the solution turns pink as it transitions into the basic pH range of the indicator.

Step-by-Step Solution

1
Identify the nature of the titrant and analyte.
Ethanoic acid is a weak acid, and sodium hydroxide is a strong base.
The equivalence point for a weak acid-strong base titration occurs in the alkaline region (pH 8 to 10).
2
Determine the color states of phenolphthalein.
Phenolphthalein is colorless in acidic solutions (pH < 8.3) and turns pink in basic solutions (pH 8.3–10.0).
Before the end point, ethanoic acid predominates so the flask is colorless. At the end point, excess hydroxide ions cause the solution to turn pink.

Key Concept

Indicator Choice and End Point Colors in Acid-Base Titrations
Estimated Time:1m 0s
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Topics

Acid-Base Titrations, Indicators, and Volumetric Calculations

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