Question

Phenolphthalein is used as an indicator for titration of sodium hydroxide solution against a standard solution of oxalic acid. The colour change observed at an alkaline pH close to equivalence point of this titration is:

• A. Colourless to pink
• B. Pinkish red to yellow
• C. Pink to colourless
• D. Yellow to pinkish red

Hint:

Phenolphthalein is colourless in acidic solution and pink in basic solution.

Answer 1

The Correct Option is A

Step 1: Understand phenolphthalein indicator.
Phenolphthalein is an acid-base indicator. It is: \[ \text{colourless in acidic medium} \] and: \[ \text{pink in basic medium} \]

Step 2: Understand the titration.
The titration is between: \[ NaOH \] and: \[ \text{oxalic acid} \] This is an acid-base titration. Near alkaline pH, phenolphthalein shows pink colour.

Step 3: Determine colour change.
When the solution changes from acidic/neutral side towards alkaline side, phenolphthalein changes from: \[ \text{colourless} \] to: \[ \text{pink} \] Therefore, the observed colour change is: \[ \text{Colourless to pink}. \]