Equilibrium

Equilibrium

Question

The conjugate base of H2PO4- is

  • PO43-

  • HPO42-

  • H3PO4

  • P2O5

Answer

B.

HPO42-

H3PO4 is a tribasic acid, thus ionising in three steps.
left parenthesis straight i right parenthesis space straight H subscript 3 PO subscript 4 space rightwards harpoon over leftwards harpoon with space space on top space straight H to the power of plus space plus space straight H subscript 2 PO subscript 4 superscript minus
left parenthesis ii right parenthesis space straight H subscript 2 PO subscript 4 superscript minus space rightwards harpoon over leftwards harpoon with space on top space straight H to the power of plus space plus space HPO subscript 4 superscript 2 minus end superscript
left parenthesis iii right parenthesis space HPO subscript 4 superscript 2 minus end superscript space rightwards harpoon over leftwards harpoon with space space on top space straight H to the power of plus space plus space PO subscript 4 superscript 3 minus end superscript

The conjugate base is formed when an acid loses its proton. Thus, HPO42- is the conjugate base of H2PO4- (which is an acid in step II, but is the conjugate base of H3PO4 in step I)

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