Since pure water is neutral in pH, what does it contain?

Question

2278 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2016-09-21T07:08:05

Shouldn't you tell us what this signifies?

Water undergoes autoprotolysis: #2H_2OrightleftharpoonsH_3O^(+) + ""^(-)OH#.

This equilibrium has been carefully measured, and under standard conditions, #K_w# #=# #[H_3O^+][""^(-)OH]# #=# #10^-14#.

For distilled water, #[H_3O^(+)] = [""^(-)OH]=10^-7*mol*L^-1#.

For the relationship,#[H_3O^+][""^(-)OH]# #=# #10^-14#, we can take #log_10# of both sides to give:

#log_10[H_3O^+]+log_10[""^(-)OH]# #=# #log_10(10^-14)#, i.e.

#log_10[H_3O^+]+log_10[""^(-)OH]# #=# #-14#, equivalently,

#-log_10[H_3O^+]-log_10[""^(-)OH]# #=# #+14#,

If we define #pH=-log_10[H_3O^+]#, and #pOH=-log_10[HO^-]#, then:

#pH+pOH=14.#

And of course neutrality requires that #[H_3O^+]=[HO^-]#, and #pH= (pOH) =7.#