Two students performed a titration to determine the concentration of a solution of hydroiodic acid (HI). They used a known solution of 0.100M potassium hydroxide (KOH) as the standard solution. Phenolphthalein is an indicator that changes from colorless to pink around pH of 8. It was chosen as an indicator for the reaction. Why was phenolphthalein a good choice?

Because the solution changes from acid to basic and the indicator show this changing the color of the solution from colorless to pink

Explanation:

Hydroiodic acid is a strong acid and potassium hydroxide is a strong base. At first, the solution only has acid, then its pH is low and the indicator is colorless. As the hydroxide is added, the pH increases slowly, and it is maintained below 7, so the solution still is colorless. After the first extra drop of KOH is added, the solution rapidly change to basic, its pH is now greater than 8, and the indicator changes the solution's color to pink