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Highly selective Backlog Qs for JEE MainChemistryIonic Equilibrium

The titration curve of weak acid vs. strong base with phenolphthalein as indicator is shown below. The K phenolphthalein = 4 × 10 - 10 Given: log 2 = 0 . 3 The number of following statement/s which is/are correct about phenolphthalein is _______ A. It can be used as an indicator for the titration of weak acid with weak base. B. It begins to change colour at pH = 8 . 4 C. It is a weak organic base D. It is colourless

Correct answer

2

Step-by-step solution

Phenolphthalein is an organic acid and can be represented as HPh . HPh (colourless) ⇌ H + + Ph - (Pink) Using Henderson equation for phenolphthalein pH = pK In + log Ph - [ HPh ] At equivalence point Ph - = [ HPh ] ∴ pH 2 = pK In = - log [ 4 ] + 10 = - 0 . 6 + 10 = 9 . 4 Hence at ( 9 . 4 ± 1 ) PH , phenolphthalein starts changing colour. Phenolphthalein is colourless in acidic medium and pink in basic medium. Phenolphthalein indicator distinguish the pH change between 8 to 10 . Therefore, it is used for strong acid

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