Question bank/NEET UG/Chemistry

NEET UGChemistryModerateNEET 2025

Phosphoric acid ionizes in three steps with their ionization constant values Ka1K_{a_1} , Ka2K_{a_2} and Ka3K_{a_3} , respectively, while K is the overall ionization constant. Which of the following statements are true?

A. logK=logKa1+logKa2+logKa3log K = log K_{a_1} + log K_{a_2} + log K_{a_3}

B. H3PO4H_3PO_4 is a stronger acid than H2PO4−H_2PO_4^- and HPO42−HPO_4^{2-}

C. Ka1>Ka2>Ka3K_{a_1} > K_{a_2} > K_{a_3}

D. Ka1=Ka3+Ka22K_{a_1} = \frac{K_{a_3} + K_{a_2}}{2}

Choose the correct answer from the options given below :

Show answer and solution
Sol. H3PO4H_3PO_4 is a stronger acid than H2PO4−H_2PO_4^- and HPO42−HPO_4^{2-}.

H3PO4(aq)⇌H+(aq)+H2PO4−(aq)Ka1=7.5×10−3H_3PO_4(aq) \rightleftharpoons H^+(aq) + H_2PO_4^-(aq) \quad K_{a_1} = 7.5 \times 10^{-3}

H2PO4−(aq)⇌H+(aq)+HPO42−(aq)Ka2=6.2×10−8H_2PO_4^-(aq) \rightleftharpoons H^+(aq) + HPO_4^{2-}(aq) \quad K_{a_2} = 6.2 \times 10^{-8}

HPO42−(aq)⇌H+(aq)+PO43−(aq)Ka3=1.7×10−12HPO_4^{2-}(aq) \rightleftharpoons H^+(aq) + PO_4^{3-}(aq) \quad K_{a_3} = 1.7 \times 10^{-12}

Ka1>Ka2>Ka3K_{a_1} > K_{a_2} > K_{a_3}

logK=logKa1+logKa2+logKa3log K = log K_{a_1} + log K_{a_2} + log K_{a_3}

Ans. (A), (B) and (C) only

Source: geekyrakshit/indian-exam-questions / neet2025-code45

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