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Ionic Equilibrium question

2004 · Shift 0 · Q3
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Ionic Equilibrium question

2004 · Shift 0 · Q3

JEE MainChemistryIonic EquilibriumMCQ+4 / −1
The conjugate base of H2PO4−H_2PO_4^-H2​PO4−​ is :
  1. A
    PO43−PO_4^{3-}PO43−​
  2. B
    HPO42−HPO_4^{2-}HPO42−​
  3. C
    H3PO4H_3PO_4H3​PO4​
  4. D
    P2O5P_2O_5P2​O5​
View written solutionFree

Correct answer: B

  1. Recall the definition of a conjugate base

    A conjugate base is formed when an acid donates one proton (H+)\left(H^+\right)(H+).

  2. Given species

    The species is H2PO4−H_2PO_4^-H2​PO4−​. If it acts as an acid, it loses one proton:

    H2PO4−→HPO42−+H+H_2PO_4^- \rightarrow HPO_4^{2-} + H^+H2​PO4−​→HPO42−​+H+

  3. Identify the conjugate base

    After losing one H+H^+H+, H2PO4−H_2PO_4^-H2​PO4−​ becomes:

    HPO42−HPO_4^{2-}HPO42−​

  4. Check options

    • A: PO43−PO_4^{3-}PO43−​ → this would be formed after losing another proton, not just one.
    • B: HPO42−HPO_4^{2-}HPO42−​ → correct.
    • C: H3PO4H_3PO_4H3​PO4​ → this is the conjugate acid, formed by gaining H+H^+H+.
    • D: P2O5P_2O_5P2​O5​ → unrelated.

  5. Final answer

    The conjugate base of H2PO4−H_2PO_4^-H2​PO4−​ is:

    HPO42−\boxed{HPO_4^{2-}}HPO42−​​

So, the correct option is B.

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