Question

Why does the pink color, which forms at the point where the NaOH comes into contact with phenolphthalein in the flask, disappear more slowly near the end point?

a) The reaction slows down as the endpoint is approached
b) Phenolphthalein is depleted near the endpoint
c) Excess NaOH slows down the reaction
d) The solution becomes too acidic near the endpoint

Answer 1

The pink color from the phenolphthalein in a titration fades more slowly near the endpoint due to a reduced concentration of weak acid leading to a larger pH change with each drop of NaOH and the approach of equal concentrations of hydronium and hydroxide ions.

Step-by-step explanation:

The pink color disappearing more slowly near the endpoint during a titration involving NaOH and phenolphthalein is due to the buffering capacity of the weak acid being titrated. As the endpoint of a titration is approached, most of the weak acid has been neutralized, and there's less acid available to react with the NaOH being added.

This means that each additional drop of NaOH has a more significant effect on the pH, causing the solution to remain pink for a longer time before disappearing as it is closer to the true endpoint. Additionally, at the endpoint, the concentration of hydronium ions (H+) and hydroxide ions (OH-) is becoming equal, making the color change more gradual as the pH changes less with the addition of each drop.