In: Chemistry
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
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