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Scoring Scheme: 3-3-2-1 Part II. You considered the properties of two acid-base indicators, phenolphthalein and methyl orange. Many indicators are weak acids in water and establish the equilibrium: HIn(aq)(Color 1) H2O(l) H3O (aq) In-(aq)(Color 2). Indicators change color depending on whether they are in a protonated (HIn) or unprotonated (In-) form. What is the equilibrium expression for the phenolphthalein indicator in water and what colors are the protonated and unprotonated forms of the indicator

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Answer:

Explanation:

Phenolphthalein is a protonated indicator and methyl orange is a basic indicator having hydroxyl ionisable part .

Phenolphthalein can be represented by the following formula

HPh  which ionizes in water as follows

       HPh        + H₂O      ⇄    H₃O⁺      +        Ph⁻

( colourless  )                    ( pink  )

In acidic solution it is in the form of protonated Hph form which is colourless

In basic medium , it ionises to give H₃O⁺ and unprotonated  Ph⁻ whose colour is pink .

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