Three reactions involving $\ce{H2PO4-}$ are given below: (i) $\ce{H3PO4 + H2O -> H3O+ + H2PO4-}$ (ii) $\ce{H2PO4- + H2O -> HPO4^{2-} + H3O+}$ (iii) $\ce{H2PO4- + OH- -> H3PO4 + O^{2-}}$ In which of the above does $\ce{H2PO4-}$ act as an acid?

JEE Main · 2010 · 11 Apr 2010 · Offline · Equilibrium

Three reactions involving

H X_{2} PO X_{4} X^{-}

are given below:
(i)

H X_{3} PO X_{4} + H X_{2} O H X_{3} O X^{+} + H X_{2} PO X_{4} X^{-}

(ii)

H X_{2} PO X_{4} X^{-} + H X_{2} O HPO X_{4} X^{2 -} + H X_{3} O X^{+}

(iii)

H X_{2} PO X_{4} X^{-} + OH X^{-} H X_{3} PO X_{4} + O X^{2 -}

In which of the above does

H X_{2} PO X_{4} X^{-}

act as an acid?

Answer: (A)

H X_{2} PO X_{4} X^{-}

acts as a Bronsted acid only when it donates a proton. In (ii) it loses

H^{+}

to give

HPO X_{4} X^{2 -}

(acid). In (i) it is the product, i.e. the conjugate base of

H X_{3} PO X_{4}

, and in (iii) it accepts a proton to form

H X_{3} PO X_{4}

(base). Hence it acts as an acid in (ii) only.

JEE Main · 2010 · 11 Apr 2010 · Offline · Single correct option