1. Phenolphthalein (Hln), which is commonly used as an indicator in acid-base ti
ID: 1031489 • Letter: 1
Question
1. Phenolphthalein (Hln), which is commonly used as an indicator in acid-base titrations, is a weak acid. In an aqueous solution, Hln exists in equilibrium with its conjugate base, ln according to the equation:
HIn(aq) + H2O(l) <=> ln- + H3O+ (aq)
colorless (deep pink)
a) Describe how the equilibrium will shift when you add concentrated HCl to the above equilibrium mixture.
b) If you carried out this experiment in the lab, what change would you look for to confirm this shift?
c) Describe how the equilibrium will shift when you add concentrated NaOH to the above equilirium mixture.
Explanation / Answer
According to Le-Chatelier's principle, if any of conditions of a chemical reaction at equilibirium changes,the reaction will shift to a direction to oppose that change and restore the equilibirium.
a) By adding HCl, it will increase the hydronium ion concentration and the reaction will be shifted to the left side and accordingly phenolphathalein ion concentration decreases.
b) Phenolphathalein molecule is colourless and phenolphathalein ion is pink in colour. Adding extra hydrogen ions shift the position of equilibirium to the left and and turns the indicator colourless.Then in lab, we can confirm the shift.
c) By adding NaOH, it will increase the water concentration so the reaction will be shifted to the right side accordingly increasing the concentration of the phenolphathalein ions. That is, Hln decreases and ln- increases.
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