Question

Indicators are used as color “sensors” for the concentration of H+ ions in a solution. Phenolphthalein changes from colorless in acid (high H+) to pink (lower H+) in base. When a solution containing carbonate ions is added to this equilibrium, the color becomes more pink. Based on the color, have the H+ ions increased or decreased? _ HCO3- + H2O ⇌ H+ + CO32- increased decreased remain same. None of these.

Answer 1

Phenolphthalein is an indicator whose working range is between pH 8-10 . at pH below 8 it is colorless and at pH 8-10 it shos change in color from colorless to pink. and at pH 10 it completely changes to pink which is also the end point of the reaction and it indicates the reaction is completed.

so the pink color appears in basic condition and solution become colorless in acidic medium

In HCO₃- + H₂O ⇌ H⁺ + CO₃^2- equillibrium as more CO₃^2- is added it consumes H⁺ ion causing decrease in H⁺ ion and according to Lechatlier principle it shifts the reaction will shift forward making it more basic. Hence the color will become more pink i.e dark pink