What does water dissociate into?

1 Answer
Truong-Son N.
Jul 6, 2016

I believe you are referring to the autoionization of water. All that really happens is a transfer of an #"H"^(+)# to another water molecule.

This is very slight, but it does happen and we have to be aware of it as it is very important to include it to determine relationships between #"pH"#, #"pOH"#, #"pK"_a#, and #"pK"_b#.

#\mathbf(2"H"_2"O"(l) rightleftharpoons "H"_3"O"^(+)(aq) + "OH"^(-)(aq))#

The equilibrium constant for this is

#color(blue)("K"_w = ["H"_3"O"^(+)]["OH"^(-)] = 10^(-14)),#

remembering to NOT include liquids and solids, as their contribution is considered #1#, and #1# times anything is itself.

This yields relationships like

#"pH"# #+# #"pOH" = 14,#

#"K"_axx"K"_b = "K"_w,#

and

#"pK"_a + "pK"_b = "pK"_w,#

where:

  • #"K"_a# and #"K"_b# are acid or base dissociation equilibrium constants, respectively.
  • #"pK"_x# is the #-log_10# of #"K"_x#.
  • #"pH" = -log["H"^(+)] = -log["H"_3"O"^(+)]#.
  • #"pOH" = -log["OH"^(-)]#.
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