(I) H₃PO₂ , (II) H₃PO₃ (III) H₃PO₄

Correct increasing order of acidic strength ( $K_{a_1}$ ) is

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III ,I , II

II , I , III

I , II , III

III , II , I

answer is D.

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Detailed Solution

structure of conjugate bases of I , II , III respectively

(A) (B) (C)

Consider (A)

Equivalent (indistinguishable) structures

maximum dispersal of negative charge

consider (B) : possibility (I)

Possibility (II)

Thus structure (B) is less stable as compared to (A)

Consider (C) : possibility (I)

Possibility (II)

Possibility (III)

Thus stability of H₃PO₂^- > H₂PO₃^- > H₂PO₄^-

Other explanation:

-I effect of P=O group is distributed only on one -OH in H₃PO₂ where as on two -OH groups in H₃PO₃

And three -OH groups in H₃PO₄

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