Question

In: Chemistry

Which ONE of the following represents the CORRECT equilibrium constant expression for the reaction: H2PO4 ¯...

Which ONE of the following represents the CORRECT equilibrium constant expression for the reaction:

H2PO4 ¯ + OH¯ ⇔ HPO42-+ H2O

(a) 1/kb(b) 1/ka (c) kw/ka(d) kw/kb(e) kb/kw

Solutions

Expert Solution

Correct option = (a)1/ Kb

Given,

H2PO4- + OH- HPO42-+ H2O

Keq = ([HPO42-][H2O])/([H2PO4- ][OH-])

      = [HPO42-]/([H2PO4- ][OH-])       ([H2O] =1   as it is a pure liquid)

Now consuder the following equilibrium reaction,

HPO42-+ H2O H2PO4- + OH-

In this case,

Equilibrium constant = Base dissociation constant (Kb)

equilibrium constant = Kb = ([H2PO4- ][OH-])/[HPO42-]

                             or Kb   = 1/ Keq                     ( Keq = [HPO42-]/([H2PO4- ][OH-])

                            or, Keq = 1/ Kb    

Therefore correct equilibrium constant expression = option (a) = 1/ Kb         


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