What is the pH of pure water at #25^@ "C"# and what is its equilibrium constant for autoionization?

1 Answer
Truong-Son N.
Sep 14, 2015

#H_2O rightleftharpoons H^(+) + OH^(-)#

This happens very rapidly, with a special equilibrium constant #K_w# of #1xx10^(-14)#.

That means in pure/de-ionized water, with the #pH# also perfectly #7# in the absence of carbon dioxide:

#K_w = [H^(+)][OH^(-)]#

#[H^(+)] = 1xx10^(-7)" M"#
#[OH^(-)] = 1xx10^(-7)" M"#

Just note that this is an ideal situation, because the carbon dioxide in the air will lower the #pH# of water exposed to the air (eventually to about #5.65#).

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