The first, second and third dissociation constants of a polybasic acid are in the order

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$K a_{1} > K a_{2} > K a_{3}$

$K a_{1} = K a_{2} = K a_{3}$

$K a_{1} < K a_{2} < K a_{3}$

$K a_{1} > K a_{2} = K a_{3}$

answer is A.

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Detailed Solution

The ionization reactions for polybasic acid are:

$H_{3} A ⇌ H^{+} + H_{2} A^{-}$ --------------- $K a_{1}$

$H_{2} A^{-} ⇌ H^{+} + H A^{2 -}$ --------------- $K a_{2}$

$H A^{2 -} ⇌ H^{+} + A^{3 -}$ --------------- $K a_{3}$

An acid is strong if its conjugate base is weak & stable.

Strength of a acid: $H_{3} A > H_{2} A^{-} > H A^{2 -}$

Strength of a base: $A^{3 -} > H A^{2 -} > H_{2} A^{-}$

& value of acid ionisation constant of a polybasic acid is of order $K a_{1} > K a_{2} > K a_{3}$

Hence, the correct option is option 1) $K a_{1} > K a_{2} > K a_{3}$ .

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